Answer
$b=-1$
Since $5$ is a zero of the function, substituting $5$ to $x$ must yield $y=0$.
Check the solution below.
Work Step by Step
Recall:
if $a$ is a zero of the function $f(x)$, then $f(a)=0$.
Hence, if $5$ is a zero of the function $y=x^2+bx-20$, then substituting $5$ to $x$ must give $y=0$:
\begin{align*}
y&=x^2+bx-20\\
0&=5^2+b(5)-20\\
0&=25+5b-20\\
0&=5+5b\\
0-5&=5b\\
-5&=5b\\
\frac{-5}{5}&=\frac{5b}{5}\\
-1&=b
\end{align*}
