X=4 
Explanation 
First subtract 4x^2 + 6x from both sides of the equation 
X^2 - 8x + 16 = 0 
Then let’s factorice the equation using the middle-term break: 
x^2 - 4x - 4x + 16 = 0
x (x - 4) - 4 (x - 4) = 0
(x - 4) (x - 4) = 0 
(x - 4) ^2 = 0 
x = 0 
Therefore the solution to the equation is x = 4