A car travels along a straight road for 30 seconds starting at time t = 0. Its acceleration in ft/sec2 is given by the linear graph below for the time interval [0, 30]. At t = 0, the velocity of the car is 0 and its position is 10.

Question

A car travels along a straight road for 30 seconds starting at time t = 0. Its acceleration in ft/sec2 is given by the linear graph below for the time interval [0, 30]. At t = 0, the velocity of the car is 0 and its position is 10. 
What is the velocity of the car when t = 6? 
I can't show the graph, but the y intercept is at (0,10) and the x intercept is (10,0).

So if the equation of the graph is 10-t, then the velocity equation would be 10t-t^2/2+C and I got C to be 0. I got 42 ft/sec to be my answer for the velocity at t=6 by plugging 6 in for t. Is this all there is to it or is there something I'm missing regarding the position formula?