Question

What is X plus 8 equals 12

Answer 1

$x+8=12 to find out what x is, subtract the numbers. 12-8=4 x=4 Checking: 4+8=12$

Answer 2

X = 4. As you need a number that when added to 8, will give you 12. And 4 is the only number able to do that. I don't know why you're asking this if you're in high school however.