3.115: Velocity–time graph: find $V$ and acceleration

The diagram shows the velocity–time graph for a particle travelling in a straight line. When $t=30$ the velocity is $V\,\mathrm{ms}^{-1}$. The particle travels $800\,\mathrm{m}$ in $45\,\mathrm{s}$. Find $V$.

Find the acceleration of the particle when $t=35$.