input and output what is the rule

Question

if the output is 8 and the rule is x time x-2 what would the input be?

Answer 1

x^2 -2x = 8 = output

complete the square

x^2 -2x +1 = 8+1

(x-1)^2 = 9

take square roots

x-1 = +/-3

x= 1 + 3 or 1-3

x = input = 4 or -2