Call the number x for now. It is a placeholder for the answer.
"A number divided by 3" translates to x/3 where the slash means division.
"A number divided by 3 is -11" means we have this equation:
x/3 = -11
To solve for x, we multiply both sides by 3. Why multiply? Because we want to undo the division operation done to x. This will get x all by itself.
Doing so leads to...
x/3 = -11
3*(x/3) = 3*(-11)
x = -33
So the unknown number is -33
As a check
x/3 = -11
-33/3 = -11
-11 = -11
so the answer is confirmed
