5n^2-3n-15=10
The coefficient of the squared term is a = 5.
If it's not obvious how to complete the square, a standard method involves multiplying evrything by 4a = 20.
If we do that we get,
100n^2 - 60n - 500 = 0
We now complete the square by writing the above as,
(100n^2 - 60n + 9) - 9 - 500 = 0
(10n - 3)^2 - 509 = 0
(10n - 3)^2 = 509