The gradient or the steepness of the graph can be used to work out the acceleration ... ie your answer must be fairly long. From 0 to 20 seconds the vehicle is at a constant speed of 12 m/s.

The gradient of a velocity time graph represents acceleration, which is the rate ... frac{7~m/s}{8~s} = 0.875~{m/}{s}^2\) Notice, that after about 10 seconds, the gradients are negative meaning ...