P= A/ l + rt
Hello,
unfortunately your equation isn't clear.
If you mean:
$\displaystyle P=\frac Al + rt~\iff~ P - \frac Al = rt~\iff~r = \frac Pt - \frac{A}{l \cdot t}$
or:
$\displaystyle P=\frac{A}{l + rt}~\iff~l+rt=\frac AP~\iff~rt = \frac AP - l~\iff~r = \frac{A}{P\cdot t} - \frac lt$