From Mechanics

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

This area can be calculated either by breaking up the area into three parts, two triangles and a rectangle or by using the formula for the area of a trapezium as the graph in the figure defines a trapezium with the \displaystyle t-axis.

This latter method gives

\displaystyle \frac{1}{2}\left( 21+\left[ 16-7 \right] \right) \times 9 \ \text{m}{{\text{s}}^{\text{-1}}}=135\ \text{m}