Question

P = a + (n - 1)b, for b

Answer 1

P = a + (n - 1)b   Subtratc a from both sides
P - a = (n - 1)b   Divide both sides by (n - 1)
$\frac{P - a}{n - 1}$ = b   Switch the sides to make it easier to read
b = $\frac{P - a}{n - 1}$

Answer 2

The answer is b=P-a-(n+1) all you had to do was the a and the parenthesis from the b and put it on the other side