Solution:
Given the expression;
[tex]x^2+18x+c[/tex]
c is the half of square of coefficient of x. That is;
[tex]\begin{gathered} x^2+18x+c=x^2+18x+(\frac{1}{2}(18))^2 \\ \\ x^2+18x+c=x^2+18x+9^2 \\ \\ x^2+18x+c=x^2+18x+81 \\ \\ x^2+18x+81=(x+9)(x+9) \end{gathered}[/tex]
Hence, the value of c is;
[tex]c=81[/tex]
