Explanation:
this is a special case of a line : a horizontal line.
what does that mean for the domain (valid x- values) and the range (valid y-values) ?
x goes just as usual from -infinity to +infinity.
by y is the special thing : it stays constant (in our case here -7) no matter what is the x-value.
the slope as the ratio
y coordinate difference / x coordinate difference
is therefore a constant
0 / x coordinate difference = 0
the equation is then
y = 0×x + y-intercept = y-intercept
this y-intercept is -7.
so, the equation is
y = -7