Answer
a) $y=5$
b) $y-5=0$
Work Step by Step
a) The equation of a line in slope-intercept form is:
$$\begin{align*}y=mx+b,\end{align*}$$
where $m$ is the slope and $b$ in the $y$-intercept.
We are given:
$$\begin{align*}
&P(0,5)\text{ belongs to the line}\\
&\text{The line is horizontal}.
\end{align*}$$
Because the line is horizontal, its equation is
$$\begin{align*}y=b,\text{ where }b\text{ constant}\end{align*}$$
We determine $b$ using the fact that the point $P$ belongs to the line:
$$\begin{align*}y=5.
\end{align*}$$
b) The equation of a line in general form is:
$$Ax+By+C=0.$$
Rewrite the equation $y=5$ in general form:
$$\begin{align*}
y-5&=0.
\end{align*}$$
c) Graph the line using the point $P(0,5)$ and another point with $y$-coordinate equal to $5$, for example $(3,5)$:
