Since there is no initial condition, the exact value of v cannot be determined, but you can set the equation up for a general evaluation of the situation.
s = 1/2a^2*v+c
First you need to subtract c from both sides to get
s-c = 1/2a^2*v
then you can just divide both sides by 1/2a^2 to get v
(2(s-c))/a^2=v
when dividing a fraction, such as 1/2, make sure to keep in mind that you're really multiplying the flipped version, so that dividing by 1/2 means multiplying by 2.
