Solution

а)Energy density (magnetic pressure) is calculated using the formula $$P=w=\frac{B^2}{2\mu_0}$$
Considering the induction to be constant throughout the entire cross-section and equal to $B=\frac{\mu_0I}{2b}$,
we get a formula for pressure:$$P=\frac{\mu_0I^2}{8b^2}$$
Hooke's law:
$$\frac{\Delta}{a}=\frac{P}{E}$$
$$\Delta=\frac{\mu_0aI^2}{8Eb^2}$$
б)$$P=\frac{B^2}{2\mu_0}$$
$$B=\sqrt{2\mu_0P}$$
$$B_1\approx10Тл$$
$$B_2\approx35Тл$$