Answer
Average force = 1590 N
Work Step by Step
We know that
$a=\frac{v_f-v_i}{\Delta t}$
$a=\frac{12m/s-0m/s}{4.0\times 10^{-3}s}$
$a=3\times 10^{3}m/s^2$
Now $F=ma$
We plug in the known values to obtain:
$F=(0.53Kg)(3\times 10^3m/s^2)$
$F=1.59\times 10^3N$
$F=1590N$