Answer:
c=81
Explanation:
Take half of b: 18/2=9
Square the result: 9²=81
This is your value of c that will result in a perfect square trinomial, which is c=81. If you were to keep going, you would do the following:
x²+18x+81=25+81
x²+18x+81=106
(x+9)^2=106 <-- Final simplification
(x+9