How do you find $x$ when there are also other variables like $P, A, t$
$$P=\frac{A}{1+xt}$$
Here's what I did, I'm not sure if I'm correct
LCD: 1 + xt
$$1+xt(P=\frac{A}{1+xt})$$ $$(1+xt)P =1+xt(\frac{A}{1+xt})$$ $$P + Ptx = A$$
Is the above correct? I couldn't continue it because I don't know what to do next.
It is ok. Note that necessarily $1+tx\neq 0$. Then, since $A=P(1+tx)$, $$P=0\Leftrightarrow P(1+tx)=0\Leftrightarrow A=0 $$ and so we cannot say nothing about $x$. Now suppose $P\neq 0$. Then $$A=P+Ptx=P(1+tx)\Rightarrow tx=\frac{A}{P}-1.$$ If $t=0$, then $\frac{A}{P}-1=0$, i.e., $A=P$ and, again, nothing to say about $x$. If $t\neq 0$, $$tx=\frac{A}{P}-1\Rightarrow x=\frac{A}{Pt}-\frac{1}{t}.$$ Therefore, $x$ depends of $A, P$ and $t$.