The condition of SHM is stated below,
Acceleration is directly proportional to negative of displacement of particle.
If ‘ $\mathrm{a}$ ‘ is the acceleration,
$\mathrm{x}$ is the displacement
Then, for Simple Harmonic Motion,
$\mathrm{a}=-\mathrm{kx}$ where $\mathrm{k}$ is constant
(a) $a=-10 x$
This is in the form of $a=-k x$
Therefore, this is SHM
(b) $\mathrm{a}=100 \mathrm{x}^{3}$
It is clear it is not SHM