By area under graph we find (add your rectangular and trianguar components not multiply) that:
$$s=ut+\frac{t(v-u)}{2}$$
now $a=\frac{v-u}{t}$ so multiplying both sides by $t^2/2$ gives $\frac{at^2}{2}=\frac{t(v-u)}{2}$ and the answer follows.
By area under graph we find (add your rectangular and trianguar components not multiply) that:
$$s=ut+\frac{t(v-u)}{2}$$
now $a=\frac{v-u}{t}$ so multiplying both sides by $t^2/2$ gives $\frac{at^2}{2}=\frac{t(v-u)}{2}$ and the answer follows.