Answer:
A = (1/2)Pa = Pa/2
You can multiply both sides by 2 to eliminate the fraction on the right side:
2A = 2(Pa/2) = Pa
Then divide both sides by a (or multiply both sides by 1/a) to leave P alone on the right side:
(2A)(1/a) = 2A/a = (Pa)(1/a) = P
Rewriting:
P = 2A/a
You could also have divided by a first then multiplied by 2; which you should do first does not matter in this example.
Explanation: