Suppose that $x = 2-t$ and $y = 4t + 7$.  If $x = -3$, what is $y$?
To find $t$, I can use the equation $x = 2-t$ and substitute $x = -3$. That gives me $-3 = 2-t$, which I can rearrange to get $t = 5$. Now that I have $t$, I can plug it into the equation for $y$. That gives me $y = 4t + 7 = 4(5) + 7 = 27$. So $y = 27$ when $x = -3$. 