let's set up an equation.
so we know the answer is -13.
1/3 of a number (our variable) can be written as 1/3n.
8 less than that would be subtracting 1/3n by 8
so we have -13 = 1/3n - 8.
To solve:
Add 8 to both sides: -5 = 1/3n
Multiply both sides by 3 (to cancel fraction): -15 = n
You can check by plugging -15 into the equation we made.