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 }