(x-1)/5 = 3/(x+1)
(x-1)/5 = 3/(x+1) cross-multiply
(x -1)(x + 1) = 15 simplify
x^2 - 1 = 15 add 1 to both sides
x^2 = 16 take the pos/neg square roots of both sides
x = ± √16
x = ± 4
