What number needs to be added to both sides of the equation in order to complete the square?

x^2+16x=18

You have to subtract 18 from both sides

x^2 + 16x = 18

x^2 + 16x (-18) = 18 - 18

x^2 + 16x - 18 is your answer : D

Answer: c = 64

In order to find the 'c' value to complete the square, you use the equation c = (b/2) ^2. Since 16 = b, c will equal 64. 16/2 = 8, and 8^2 = 64. The final resulting equation will be (x+8) ^2 = 82.