An electron had a constant acceleration of +3.2m/s^2. At a certain instant its velocity is +9.6m/s. What is its velocity at a)2.5 s earlier and b )2.5s later?

Question

I will be happy to critique your work.

I could a= change in velocity / change in time ?

Here's the question:
"An electron had a constant acceleration of +3.2m/s^2. At a certain instant its velocity is +9.6m/s. What is its velocity at a)2.5 s earlier and b )2.5s later?"
a=+3.2m/s^2
(1) v(t)=a*t
You should be able to answer the question with (1)
I suppose we're supposed to have the initial velocity =0, but an electron is never at rest.

so the velocity would be -8m/s for part

and 8m/s for part b?

No, I think the question wants you to suppose that t=3 and that the electron has been accelerated from rest. (That's why I said the question is somewhat imaginary. No electrons are at complete rest.)
The equation v=a*t is a line with slope a; t is the independent variable here.
At t = 3 v=a*t=+3.2*3=+9.6
v=a(t+2.5) 2.5s later
v=a(t-2.5) 2.5s earlier

Answer 1

2.5s later= 17.6
2.5 s earlier = 1.6

Answer 2

Where did you get t = 3 from? You cannot just make that assumption.