we have that
2x² + 8x - 12 = 0
Group terms that contain the same variable, and move the constant to the opposite side of the equation
(2x² + 8x)=12
Factor the leading coefficient
2*(x² + 4x)=12 -------> divide by 2-----> (x² + 4x)=6
Complete the square. Remember to balance the equation by adding the same constants to each side
(x² + 4x+4)=6+4
(x+2)²=10
the constant on the right side of the equation is 10