Possibly the reason why the kids did poorly on question 19 is that the slope isn’t negative. As x goes up, y goes up; it is a positive slope. All four of your possible answers had a negative slope! The correct answer, I think, should be y=(3/5)x-6
y=bx+c

3=15b +c; c= 3-15b

6=20b +c; 6=20b + 3-15b; 6-3=5b; 3/5=b

9=25b +c;

c=3-15b= 3-9=-6; c=-6

equation s/b y=(3/5)x-6

check 25(3/5) -6 = 15-6=9√

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