16y^{2}-40y+?
I really need to figure out what the question mark is, but I am so confused.
You are probably studying completing the square.
Study the result of something like
(5x-6)^2
the result is 25x^2 - 60x + 36
notice if you take the square roots of the first and last numbers, multiply them and double you get the middle number.
now reverse that thinking for your given case
16y^2 - 40y + ????
Did you get 25?