From Mechanics

The area under the velocity-time graph gives the distance travelled.

This area can be calculated using the formula for the area of a trapedium as the graph in the figure defines a trapezium with the \displaystyle t-axis.

This gives

\displaystyle \frac{1}{2}\left( 180+\left[ 120-40 \right] \right) \text{s}\times 24 \text{m}{{\text{s}}^{\text{-1}}}=3120\ \text{m}