Write an equation in slope-intercept form of the line that passes through (1, −9) and (−3, −9)

Little time saver tipnotice that ur points have the same y value. This means that u have horizontal lines. Horizontal lines have a slope of 0 and are represented by y = a number..with that number being ur y coordinate in ur points.
So ur equation is : y = 0x - 9 <== that is the slope intercept formbut a lot of time, u see it written as y = -9

learn this...it really helps
HOY -- horizontal lines, 0 slope, represented by y = a number
VUX -- vertical lines, undefined slope, represented by x = a number

These are the multiples of 15 but what you are probably asking or what are the factors of 15