Find the sum of all values of $k$ for which $x^2 + kx - 9x + 25 - 9$ is the square of a binomial.
x^2 + (k -9)x + 16
We have either
x^2 + 8x + 16 = ( x + 4)^2 or
x^2 -8x + 16 = ( x- 4)^2
k - 9 must be = to 8 or -8
k - 9 = 8 k - 9 = -8
k = 17 k = 1
Sum of possible k's = 17 + 1 = 18
+84 
