Answer
$b=\pm 18$
Work Step by Step
$x^{2}+bx+81$
By following the perfect-square trinomial rule $(A+B)^{2}=A^{2}+2AB+B^{2}$,
we see that $A^{2}=x^{2}$ and $B^{2}=81$.
The square roots of $A^{2}$ and $B^{2}$ are $\pm x$ and $\pm 9$.
Because of the perfect-square trinomial rule,
$ bx=2\cdot x\cdot B \qquad$..substitute $9$ for $B$ and divide the expression with $x$.
$b=2\cdot\pm 9$
$b=\pm 18$
