Answer
$s=0.730m$
Work Step by Step
According to the principle of work and energy
$\frac{1}{2}mv_1^2-U_{s_1}-U_{s_2}=\frac{1}{2}mv_2^2$
$\implies \frac{1}{2}mv_1^2-\frac{1}{2}k_1s^2-\frac{1}{2}k_2s^2=\frac{1}{2}mv_2^2$
We plug in the known values to obtain:
$\frac{1}{2}(20)(2)^2-\frac{1}{2}(50)s^2-\frac{1}{2}(100)s^2=0$
This simplifies to:
$s=0.730m$
