I don't think you can do 2.8/20, as the car is accelerating. s = d/t only works for constant speed. If they give you the acceleration, you could use the acceleration equation. If they tell you that the acceleration is constant, you could use a SUVAT formula. If they give you the equation of the curve, you could differentiate. If not, you could draw on a tangent to the curve when x = 20, and find the gradient of the curve, although this is less accurate.
Please correct me if you think that's wrong! But I'm pretty sure you can't just do s = d/t