Well we know that it has to square under 56, and the closest square to 56 is 49, which is 7 squared. Let's try that.
The sum of a number (7+) and it's square (7^2) is (=) 56.
Our equation is 7^2 + 7 = 56.
7^2 is 49.
49 + 7 = 56.
Your number is 7.
I hope this helps!