changing the subject of the formula to r means we want to get r on one side of the equal sign by itself. a simple enough concept, in practice, it can be not so simple.
P = ∏r + 2r + 2a ..............first step, get rid of the 2a from the right
=> P - 2a = ∏r + 2r ..........i subtracted 2a from both sides
=> P - 2a = r(∏ + 2) ........i factored the r out
=> (P - 2a)/(∏ + 2) = r ....i divided both sides by (∏ + 2)
so r = (P - 2a)/(∏ + 2)