Two ways to do it.
First way: Using point slope formula:
y - y1 = m(x - x1) where (x1,y1) = (8,-6)
y - (-6) = 0(x - 8)
y + 6 = 0
y = -6
Second way: Since the slope is 0, the line is horizontal, so the
graph is the black line below:
and since it goes through the y-axis where y is -6, its equation
is y = -6, because all points om that line have their y-coordinate
as -6. So you can just ignore the x-coordinate.
Edwin