I believe the opposite of a number would be the negative of that number. Haven't seen it worded like that before though. Five less is minus 5. Three times the sum is 3(x+(-7)). Is would be equals.
x= a number
Equation:
-x - 5 = 3(x + (-7))
If you solve for x:
-x - 5 = 3(x + (-7))
-x-5=3(x-7)
-x-5= (3*x) + (3*-7)
-x-5= 3x-21
Add -x to both sides
-5= 4x-21
Add 21 to both sides
16= 4x
Divide both sides by 4
4= x
Check:
-x - 5 = 3(x + (-7))
-(4)-5= 3((4) + (-7))
-9 = (3*4) + (-21)
-9 = 12 - 21
-9 = -9
Hope this helps! :)