4^x - 2^x = 56
(2^2)^x - 2^x = 56
2^(2x) - 2^x - 56 = 0
(2^x)^2 - 2^x - 56 = 0
Let u = 2^x so u^2 = (2^x)^2
u^2 - u - 56 = 0 factor
(u - 8) ( u + 7) = 0
So either
u - 8 = 0 or u + 7 = 0
u = 8 u = - 7
Substitute back
2^x = 8 2^x = - 7 this isn't possible
x = 3
Check
4^3 - 2^3 = 56 ??
64 - 8 = 56 correct !!!