What constant k will make y^2 + 42y + 16 + k the square of a binomial?
Take 1/2 of 42 = 21
Square it = 441
k = 441 -16 = 425
Proof
y^2 + 42y + 16 + 425 =
y^2 + 42y + 441 =
(y + 21)^2
