3x2−36=0
3x2=36
x2=12
x=±12
x=±23
x=−23 and 23
Although a bit tedious, we can substitute the values in the answer choices into the given equation. The first one works, so we can stop there:
3x2−36=0
3(±23)2−36=0
3(4⋅3)−36=0
3(12)−36=0
36−36=0 ✓