Answer
$4.5\times10^{-23}\space J$
Work Step by Step
Here we use the equation $W=\frac{1}{2}Kx^{2}$, Where W- work required to shrink the DNA strand, K- spring constant, x- shrunk distance of the strand.
$W=\frac{1}{2}Kx^{2}$
Let's plug known values into this equation.
$W=\frac{1}{2}\times 1.6\times10^{-7}\space N/m\space \times (\frac{2.35\space \mu m}{100})^{2}$
$W= 0.815\times10^{-7}\space N/m\space \times (\frac{2.35\times10^{-6}\space m}{100})^{2}$
$W= 0.815\times10^{-7}\space N/m\space \times 5.523\times 10^{-16}\space m^{2}$
$W= 4.5\times10^{-23}J$
