Method to find a least positive number that should be added to 1515 to become a perfect square

Probe to find the smallest perfect square that's greater than 1515:

square root of 1515 = 38.923 = = > next perfect square is the square of 39.

39 squared = 1521

6 must be added to 1515

One simple method ... Find a number whose square is nearest to 1515.

Starting with 30, its square is 900

if 40, square its 1600

and 39, square is 1521

So just by this we can say, by adding 6 to 1515, the number becomes a perfect square.