Why? Because the point (3,9) is labeled as x1 and y1 and the point (1,1) is labeled as x2 and y2, which means that the equation is set up as 1-9 / 1-3, which equals -8 / -2 , which turns out to be 4 when it is simplified. :)
First you would find how many quarts over all she has which is 6, then multiply that by 32(which is the amount of ounces are in a quart), then you would get 192 which you divide by 8 to find the total amount of cups you would have which is 24.