A block of mass m carrying a net positive charge q is positioned on...

Force on the block, $F = qE$

Acceleration of the block, $a = \dfrac{qE}{m}$

Let $t$ be the time taken by the block to reach the wall. Using the second equation of motion:

$d = \dfrac{1}{2}at^2$

$t = \sqrt{\dfrac{2d}{a}} = \sqrt{\dfrac{2md}{qE}}$

Since the collision with the wall is elastic, the block rebounds with the same speed. The electric force continues to act towards the right, retarding the block's motion to the left. It will take the same time $t$ to come to rest at its initial position.

Total time period of oscillation, $T = 2t = 2\sqrt{\dfrac{2md}{qE}} = \sqrt{\dfrac{8md}{qE}}$

For a motion to be simple harmonic, the restoring force must be proportional to the displacement ($F \propto -x$). Here, the force is constant ($qE$) and always directed towards the right. Therefore, the motion is oscillatory but not simple harmonic.