If a line is perpendicular to the y-axis, it means that the slope is zero, in the graph it will be represented by a horizontal line.
Using the slope intercept form you can determine that:
y=b+mx
if m=0
Then y=b+0*x
y=b
This expression means that the value of y will be the same regardles the value x takes.
This particular line crosses the point (0,-9), and since we know that y will be the same number then we can determine the equation to be
y=-9 for the line perpendicular to the y-axis that crosses point (0,-9)
