$$\frac{1}{f}=\frac{1}{u}+\frac{1}{v}$$
$f=\frac{uv}{u+v}$
$v\neq 0\text{ and }u\neq 0\text{ and }u\neq -v$
$u=-\frac{fv}{f-v}$
$v\neq 0\text{ and }f\neq 0\text{ and }f\neq v$