Energy stored in a spring is given by

$\mathrm{U}=\left(\frac{1}{2}\right){\mathrm{kx}}^2$ ..........(1)

Where $\mathrm{k}$ is the spring constant

Now, applying Hooke's law, we have

Tension, $\mathrm{T}=\mathrm{kx}$ (in magnitude)

$\Rightarrow$ extension $\mathrm{x}=\frac{\mathrm{T}}{\mathrm{k}}$

Substituting the value in equation (1), we get

$\mathrm{U}=\frac{1}{2}\frac{{\mathrm{T}}^2}{\mathrm{k}}$ .