Answer
$B = \frac{\mu_0}{4R} (I_2 - I_1)$
Work Step by Step
This exercise can be easily computed thanks to te results from exercise 28.34.
$B_1 = - \frac{\mu_0 I_1}{4R}$
$B_2 = \frac{\mu_0 I_2}{4R}$
$B = B_1 + B_2 = \frac{\mu_0}{4R} (I_2 - I_1)$
