Find the Algebra Mistake?

Question

Solve the following equation for x: 
log4 x + log4 (x – 3) = 1 

Answer 1

Apply formula : 

x² - 3x - 4 = 0

x² - 4x + x - 4 = 0

x(x - 4) + 1(x-  4) = 0

(x - 4)(x +1)= 0

(x - 4) = 0  and  (x +1)= 0

x = 4  and x = -1

The solution is x =  { -1 , 4 }