Answer: 81
Step-by-step explanation:
We take half of the x coefficient (18) to get 18/2 = 9
Then we square 9 to get 9^2 = 9*9 = 81
This process is done because (x+b)^2 = x^2+2bx+b^2
Notice the coefficient of the x term is 2b. That cuts in half to b, which then squares to b^2
Going back to the current problem, if c = 81 then,
x^2+18x+c = x^2+18x+81 = (x+9)^2