Let
be the unknown number. So, three times that number means
, and the square of the number is 
We have to sum 528 and three times the number, so we have 
Then, we have to subtract this number from
, so we have

The result is 120, so the equation is

This is a quadratic equation, i.e. an equation like
. These equation can be solved - assuming they have a solution - with the following formula

If you plug the values from your equation, you have

So, the two solutions would be


But we know that x is positive, so we only accept the solution 