In simple harmonic motion (SHM), total energy (E) is constant, defined as E = PE + KE, where PE is potential energy and KE is kinetic energy.
For any displacement (x) from the equilibrium position:
PE = $\frac{1}{2}kx^2$
KE = $\frac{1}{2}k(A^2 - x^2)$
When PE = KE: $\frac{1}{2}kx^2 = \frac{1}{2}k(A^2 - x^2)$
$x^2 = A^2 - x^2$
$2x^2 = A^2$
$x = \frac{A}{\sqrt{2}}$