(To write square I will use ^2 notation here)
12x^2 - 48 = 0
Add 48 to both sides
12x^2 = 48
Divide both sides by 12
x^2 = 4
Square root both sides
x = square root of 4
x = 2 or x = -2
ANOTHER SOLUTION (edit, done later)
12x^2 - 48 = 0
divide both sides by 12
x^2 - 4 = 0
(x - 2) * (x + 2) = 0
therefore
x-2 = 0 or x+2 = 0
x = 2 or x = -2