A = P(1 + rt)
Divide each side by P
A/P = P/P(1 + rt)
A/P = 1+rt
Subtract 1 from each side
A/P -1 = 1+rt-1
A/P -1 = rt
Get a common denominator
A/P - P/P
(A-P)/P = rt
Divide each side by t
(A-P)/Pt = rt/t
(A-P)/(Pt) = r
