Can’t solve Physics Question - Past Paper

A car accelerates from rest to a speed of 26m/s. The speed of the car varies over 30s:
Time: 0, 5, 10, 15, 20, 25, 30
Speed:0,16.5,22.5,24.5,25.5,26,26

First question asked me to plot a graph for it, which I did perfectly. But then it’s asking:

Sketch a graph to show how the resultant force acting on the car varies over the first 30s of motion.
Explain the shape of your graph, with reference to the graph you plotted in the first question.

HELP!!