Answer
$X_m=86.6cm$
Work Step by Step
From law of conservation of energy total energy is conserved. That is $E=P.E+K.E=200+125=375J$
Then at turning point
$E=\frac{1}{2}KX_m^2$ where $'X_m'$ is the amplitude
$X_m^2=\frac{2E}{K}$
Take square root on both sides
$X_m=\sqrt\frac{2E}{K}$
put the values
$X_m=\sqrt\frac{2(375)}{1000}$
$X_m=0.866m$
or
$X_m=86.6cm$
