Answer
$$a=-\frac{bP}{P-4};\quad \:P\ne \:4$$
Work Step by Step
We solve the problem by isolating the variable. Doing this, we find:
$$ P\left(a+b\right)=4a \\ aP-4a=-bP\\ a\left(P-4\right)=-bP\\ a=-\frac{bP}{P-4};\quad \:P\ne \:4$$
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