Complete the square to write c(x) = x2 - 16x 84 in vertex form.

Group
c(x)=(x²-16x)+84
take 1/2 of -16 and square it
-16/2=-8, (-8)²=64
ad negative and positive inside
c(x)=(x²-16x+64-64)+84
factor
c(x)=((x-8)²-64)+84
distribute or get rid of paenthsees
c(x)=(x-8)²-64+84
add
c(x)=(x-8)²+20