From Mechanics

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

This area can be calculated by breaking up the area into three parts, two triangles and a rectangle

This method gives (we ignore units in the calculation)

\displaystyle \frac{1}{2}\left( 3\times 2 \right)+\left( \left[ 7-3 \right]\times 2 \right)+\frac{1}{2}\left( \left[ 9-7 \right]\times 2 \right)=13\text{ m}