The potential energy of a particle $\left(U_x\right)$ executing SHM is given by

PE of body in SHM at an instant
 $\text{PE=U}=\frac{1}{2}m{\omega }^2{y}^2=\frac{1}{2}K{y}^2$

m is mass of oscillator, $\omega$ is angular velocity, y is displacement of oscillator, k is force constant
If the displacement , $y=\left(a-x\right)$ then
 $U=\frac{1}{2}K{\left(a-x\right)}^2=\frac{1}{2}K{\left(x-a\right)}^2$