Answer
$a=0.0476m/s^2$
Work Step by Step
We can determine the required acceleration as follows:
$\Sigma F_s=m\frac{dv}{dt}-v_{D/e}\frac{dm_e}{dt}$
We plug in the known values to obtain:
$0=(10\times 10^3+500)a-(10)(50)$
This simplifies to:
$a=0.0476m/s^2$