Question:

The acceleration of a particle is given by a = 3x+1 (where a is acceleration, x is displacement from origin). If the particle is initially at the origin and moving at a velocity of , find its velocity after 3s.

What I've done:

when t=0, x=0, v = 15

I'm not sure where to go on from here. I know if I find t in terms of x, I can substitute it into that equation. I've tried but it doesn't work well.