a
\(U =\frac{1}{2} k x^2\)

\(\text { for } x =2\)

\(U =\frac{1}{2} k (2)^2........(1)\)

\(U ^{\prime}=\frac{1}{2} k (8)^2..........(2)\)

\(\text { Eq. (2)/eq. (1) }\)

\(\Rightarrow \frac{U^{\prime}}{U^{\prime}}=\left(\frac{8}{2}\right)^2\)

\(\Rightarrow U^{\prime}=16\,U\)