Let's call this number 'x'.
We need to set up an equation using x:
x/-3 = 15
Now we just need to rearrange the equation.
Multiply both sides by -3:
x = -45
The number is -45. We can check if this is right by dividing it by -3 and checking if it is 15:
-45 / -3 = 15
Therefore -45 is correct.