$v = 0 + 2 \times 10 = 20\,m/s$

and distance travelled by it in $10 \,sec$

${S_1} = \frac{1}{2} \times 2 \times {(10)^2} = 100\;m$

then it moves with constant velocity (20 \,m/s)for $30\;\sec $

$\;{S_2} = 20 \times 30 = 600\;m$

After that due to retardation $(4\,m/{s^2})$ it stops

${S_3} = \frac{{{v^2}}}{{2a}} = \frac{{{{(20)}^2}}}{{2 \times 4}} = 50\,m$

Total distance travelled ${S_1} + {S_2} + {S_3} = 750\,m$