Answer: + or - in between the 3x and 6? Can be either 7 or 3
Explanation:
3x+6=15.
Subtract '6' on both sides.
3x=9.
Divide by '3'.
x=3
3x-6=15.
Add '-6' on both sides.
3x=21
Divide by '3'.
x=7
Again, it solely depends on your sign in between the first two numbers.