A particle is moving along a straight line OA with constant acceleration 2ms-2. At OA the particle is moving towards A with speed 5.5ms-1. The distance OA is 20m. Find the time the particle takes to move from O to A.

S=20 V=5.5 A=2 T=?

I used the formula $\displaystyle s=vt-1/2at^2 $ but I got the wrong answer.

Any help?