4x = 3 + 18 (4-x)
(4x) 4x = 3 (4x) + 18 (4-x) (4x)
(4x)2 = 3 (4x) + 18
(4x)2 - 3 (4x) - 18 = 0
(4x + 3) (4x - 6) = 0
4x + 3 = 0 or 4x - 6 = 0
4x = -3 or 4x = 6
ln(4x) = ln(-3) or ln(4x) = ln(6)
x ln(4) = ln(-3) or x ln(4) = ln(6)
x = ln(-3) / ln(4) or x = ln(6) / ln(4)
x = no real solution or x = ln(6) / ln(4)
So x = ln(6) / ln(4) is the only real solution.