Multiply both sides by `(x + 3)^2`:
`x + 5 = A(x + 3) + B`
Equating `x` parts and constant parts:
`1 = A`
`5 = 3A + B`
So `B = 2`.
Don't believe it is true! Add the right hand side and convince yourself about the accuracy of this process. You actually learn a lot about how it works by doing this. And it's always good to check your work!