If you would like to find the number which belongs in the blank to complete the square, you can calculate this using the following steps:
f(x) = 5(x^2 - 4x + a) + 15
a ... the number = ?
x^2 - 4x + a = (x - 2)^2 = x^2 - 4x + 4
a = 4
The correct result would be 4.
