An electron has a constant acceleration of $2.5 \text{ m s}^{-2}$. At a certain instant its velocity is $12.1 \text{ m s}^{-1}.$
(Indicate the direction with the sign of your answer.)
$(a)$ What was its velocity $2.5$ s earlier?
$(b)$ What is its velocity $2.5$ s later?
I'm not sure how to proceed. I know that $a=\dfrac{\Delta v}{\Delta t}$, but is it really a matter of plugging in and dividing?
\begin{align*} a &= \frac{v-u}{t} \\ v &= u+at \end{align*}
$(a) \quad $ At $t=+2.5$ s, $$v=+12.1+2.5(+2.5)$$
$(b) \quad $ At $t=-2.5$ s, $$v=+12.1+2.5(-2.5)$$