I have the following problem. I was trying to use the uncover command within an equation in my presentation. Is there any command which can do the opposite?
I try to explain my self better. Suppose I have an equation like the following:

Basically what I would like is to presente an equation contains only x_{1} and 3x_{2}, leaving 6x_{3} hidden and then to cover 3x_{2} and showing up 6x_{3}.
I have inserted the equation in a very simple way:
\[
x_{1}+3x_{2}+6x_{3}=15
\]
Does someone have an idea about how to do this? I hope I was clear.
Best wishes.
Dario
