The area under $a-t$ graph gives change in velocity.

Given, $u=-5 \,m / s$

$\Rightarrow$ Area on positive side $=\frac{1}{2} \times 6 \times 10=30 \,ms ^{-1}$

$\Rightarrow$ Area on negative side $=\frac{1}{2} \times 2 \times 10=10 \,ms ^{-1}$

Net area $=30-10=20 \,ms ^{-1}$

$\Delta v=$ Area

$v-(-5)=20$

$\Rightarrow v=15 \,ms ^{-1}$