$\frac{1}{2} \times 10 \times {18^2} = 12v + \frac{1}{2} \times 10 \times {\left( {12} \right)^2}$
or $12v = \frac{1}{2} \times 10 \times \left[ {{{\left( {18} \right)}^2} - {{\left( {12} \right)}^2}} \right]$
$ = \frac{1}{2} \times 10 \times \left[ {\left( {18 + 12} \right)\left( {18 - 12} \right)} \right]$
$12v = \frac{1}{2} \times 10 \times 30 \times 6$
or $v = \frac{{1 \times 10 \times 30 \times 6}}{{2 \times 12}} = 75\,m/s$