Fonts in the computer modern family are very light by design, so most other font choices are bolder, but here I have forced the Times-like TeX Gyre Termes and Stix Two Math fonts to be adjusted to be bolder (but less bold than the designed bold font)
Credit to mathpix for ocr your image to error free LaTeX
\documentclass{article}
\usepackage{unicode-math}
\setmainfont{TeX Gyre Termes}[FakeBold=1]
\setmathfont{Stix Two Math}[FakeBold=1]
\begin{document}
\textbf{Example}
In particular, if $X_A\left(t, t_0\right), X_A\left(t_\theta, t_0\right)=I, X_B\left(t, t_0\right), X_B\left(t_0, t_0\right)=I$, are fundamental matrix solutions of $\dot{x}=A(t) x, \dot{y}=B(t) y$, respectively, then
[
\begin{aligned}
\left|X_A\left(t, t_0\right)-X_B\left(t, t_0\right)\right| \leqq & \sup {t_0 \leqq u \leq t}\left|X_B\left(u, t_0\right)\right|\left(\exp \int{t_0}^t|A(s)| d s\right) \
& \times \int_{t_0}^t|A(s)-B(s)| d s
\end{aligned}
]
for all $t \geqq t_0$. Relation (1.11) implies that the principal matrix solution of $\dot{x}=A x$ is a continuous function of the integrable matrix functions $A$ defined on $[0, t]$ with $|A|=\int_0^t|A(s)| d s$.
When a linear differential equation contains general time varying
coefficients, the remarks in this section essentially comprise the
theory concerning the specific structure of the solutions. For special
equations, much more detailed information is available. For equations
with constant or periodic coefficients, the general structure of the
solutions is known and discussed in Sections 4 and 7.
\end{document}
For xelatex you may need to reference the fonts by filename so
\setmainfont{texgyretermes-regular.otf}[FakeBold=1]
\setmathfont{STIXTwoMath-Regular.otf}[FakeBold=1]
