How to align 2 tikzpictures, functions in 1 line with captions?

Question

I need to place both of these funcionts in 1 line, side by side with caption beneith, I tried all options, and to combine them, nothing worked. When I try to use minipage to divide 1 line to two parts, it says like it can't pair up with figure and tikz bellow, or something like that, can't make them in 1 line.. Any thoughs? Tnx for help.

\documentclass[a4paper,10pt]{article}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{calc,arrows,shapes,automata,background,petri,patterns,shadows,fadings,decorations,decorations.text,circuits.logic.US}
\usepackage{circuitikz}
\usepackage{ifthen}
\begin{document} 
\begin{figure}[h]
\begin{tikzpicture}[scale=0.75]
     \begin{axis}
    [grid=major,xmin=-5,xmax=5,ymin=-20,ymax=20 ,xlabel=$x$,ylabel=$y$]
        \foreach \a [count=\xi] in {-2.4,-2.1,...,2.4}
        { 
        \ifodd\xi           
            \addplot[blue!100,line width=1pt]  plot expression{\a*(\x^2)-3*\x+cos(90*\x)};
        \else
            \addplot[orange!100, dashed, line width=1pt] plot expression    {\a*(\x^2)-3*\x+cos(90*\x)};        
        \fi }                   
    \end{axis}
\end{tikzpicture}
\caption{Serija složenih funkcija}
\begin{tikzpicture}[scale=0.75]
\begin{axis}
[grid=major,xmin=0,xmax=3,xlabel=$t$,ylabel={$x(t),y(t)$},variable=\t]  
\addplot [blue,line width=1pt,smooth] plot [domain=0:3.5,samples=350]expression{(1+2*t)*exp(-2*t)*sin(280*t*3.14)}; 
\addplot [red, line width=1pt,smooth,style=dashed] plot [domain=0:3.5,samples=150] expression{(1+2*t)*exp(-2*t)};
\addplot [red, line width=1pt,smooth,style=dashed] plot [domain=0:3.5,samples=150] expression{-(1+2*t)*exp(-2*t)};   
\end{axis}
\end{tikzpicture}
\caption{Damping sinusne funkcije}
\label{damping}
\end{figure}
\end{document}

this is how my document looks like and the picture bellow is what it need to be like, don't know how to align them.

Answer 1

The minipages are needed to center the captions. I also had to reduce the scale slightly to get the tikzpictures to fit.

\documentclass[a4paper,10pt]{article}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{calc,arrows,shapes,automata,petri,patterns,shadows,fadings,decorations.text,circuits.logic.US}
\usepackage{circuitikz}
\usepackage{ifthen}
\usepackage{showframe}% debugging tool

\begin{document} 
\begin{figure}[h]
\hfil\begin{minipage}[t]{0.45\textwidth}
\centering\begin{tikzpicture}[scale=0.6]
     \begin{axis}
    [grid=major,xmin=-5,xmax=5,ymin=-20,ymax=20 ,xlabel=$x$,ylabel=$y$]
        \foreach \a [count=\xi] in {-2.4,-2.1,...,2.4}
        { 
        \ifodd\xi           
            \addplot[blue!100,line width=1pt]  plot expression{\a*(\x^2)-3*\x+cos(90*\x)};
        \else
            \addplot[orange!100, dashed, line width=1pt] plot expression    {\a*(\x^2)-3*\x+cos(90*\x)};        
        \fi }                   
    \end{axis}
\end{tikzpicture}
\caption{Serija složenih funkcija}
\end{minipage}\hfil
\begin{minipage}[t]{0.45\textwidth}
\centering\begin{tikzpicture}[scale=0.6]
\begin{axis}
[grid=major,xmin=0,xmax=3,xlabel=$t$,ylabel={$x(t),y(t)$},variable=\t]  
\addplot [blue,line width=1pt,smooth] plot [domain=0:3.5,samples=350]expression{(1+2*t)*exp(-2*t)*sin(280*t*3.14)}; 
\addplot [red, line width=1pt,smooth,style=dashed] plot [domain=0:3.5,samples=150] expression{(1+2*t)*exp(-2*t)};
\addplot [red, line width=1pt,smooth,style=dashed] plot [domain=0:3.5,samples=150] expression{-(1+2*t)*exp(-2*t)};   
\end{axis}
\end{tikzpicture}
\caption{Damping sinusne funkcije}\label{damping}
\end{minipage}
\end{figure}
\end{document}