The spring is unstretched at the moment a man starts pulling on the...

Let the equilibrium position of the block be at a displacement $x$ from its initial unstretched position.

At the equilibrium position, the net force acting on the block is zero. The constant force $F$ applied by the man is balanced by the restoring force of the spring.

$F = kx$

$\Rightarrow x = \dfrac{F}{k}$

The energy stored in the spring at this position is the elastic potential energy, given by:

$U = \dfrac{1}{2}kx^2$

Substituting the value of $x$:

$U = \dfrac{1}{2}k$\left(\dfrac{F}{k}\right)^2$<br><br>$U = $\dfrac{1}{2}$k\left(\dfrac{F^2}{k^2}\right)

$U =$\dfrac{F^2}{2k}