Solution

In Problem 9.3.17, a formula was obtained for the induction inside the plate. On its axis, it is zero and increases linearly with distance from the plate: $$B=\mu_0 j x$$

This means that, moving along the x-axis, only the transverse plate makes a contribution, and when we leave it, the induction becomes constant and equal to $$B_{max}=\frac{\mu_0 j h}{2}$$