Answer
$$ y=5.$$
Work Step by Step
Since the line is horizontal, its slope is $0$; hence the line has the equation
$$ y=(0)x+c=c.$$
To find $ c $, we make use of the fact that the line passes through $(-3,5)$:
$$5= c\Longrightarrow c=5$$
Hence the equation of the line is
$$ y=5.$$
