This is how we use exponents and Superscript in Mathjax
$$v(t)= ( 1-e^{-\frac{t}{RC}}).V $$
Notice that you need to include the whole thing in curly bracket. Notice that this one does not include everything in the superscript

$$e^-\frac{t}{RC} $$

And this one does

$$e^{-\frac{t}{RC}} $$