By factoring and by completing the square

Ax^2 + bx + c

To factor, find two numbers that add up to your b term (-4) but also multiply to get your c term (-12)

That's how I got +2 and -6

2-6=-4

2 x -6 = -12

(x+2)(x-6)

Factoring
what 2 numbers multiply to get -12 and add to get -4?
-6 and 2
(x-6)(x+2)=0
set to zero
x-6=0
x=6
x+2=0
x=-2

x=-2 and 6

complete the square
isolate x terms
(x^2-4x)-12=0
take 1/2 of -4 and square it ((-2)^2=4)
now add positive and neagtive inside
(x^2-4x+4-4)-12=0
factor perfect square
((x-2)^2-4)-12=0
get rid of parenthasees
(x-2)^2-4-12=0
(x-2)^2-16=0
add 16 both sides
(x-2)^2=16
sqrt both sides, don't forget positive an dneagit ve roots
x-2=+/-4
add 2 to both sides
x=2+/-4
x=2+4=6
x=2-4=-2

x=-2 and 6