If $\frac{x + 7}{(x - 1)(x + 3)} = \frac{A}{x - 1} + \frac{B}{x + 3}$, what is $B$?

Explanation

1. 01

   $x + 7 = A(x + 3) + B(x - 1)$.

2. 02

   Put $x = -3$: $4 = -4B$, so $B = -1$. (And $x = 1$ gives $A = 2$.)