Answer
Please see the work below.
Work Step by Step
We know that
$U=\frac{1}{2}Kx^2$
We plug in the known values to obtain:
$U=\frac{1}{2}(0.046\frac{pN}{\mu m})(26\mu m)^2$
$U=\frac{1}{2}(0.046\times \frac{1\times 10^{-12}N}{1\times 10^{-6}m})(26\times 10^{-6}m)^2$
$U=1.6\times 10^{-17}J$
