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