Answer:
[tex]1.5\:\mathrm{J}[/tex]
Explanation:
The elastic potential energy of a spring is given by the following:
[tex]PE_s=\frac{1}{2}k\Delta x^2[/tex], where [tex]k[/tex] is the spring constant and [tex]\Delta x[/tex] is displacement.
The displacement of the string is [tex]0.50-0.40=0.10\: \mathrm{m}[/tex].
Plugging in our given values, we get:
[tex]PE_s=\frac{1}{2}\cdot300\cdot0.10^2=\fbox{$1.5\:\mathrm{J}$}[/tex].
