Answer
$$\color{blue}{\bf\text{The slope of every horizontal line is 0}}$$
Work Step by Step
We are told we have a horizontal line passing through point $\bf{(5,1)}$ and asked to find the slope.
Recall that standard form is:
$$\bf{y=ax+b}$$
where $\bf{b}$ is the $\bf{y}$ intercept, and
$\bf{a}$ is the slope
Horizontal lines have a single $\bf{y}$ value for every $\bf{x}$;
and a $\bf{\frac{rise}{run}}$ of $\bf0$
$\bf{y=0x+b}$
or
$\bf{y=b}$
Our $\bf{y \text{ intercept}}$ is $\bf1$, but we are only asked for the slope which is $$\color{blue}{\bf\text{0 for every horizontal line}}$$
