Help!! What’s c?? I can’t got it

 81

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