I have been stuck on this problem for a while need help on it and soon. It's functions.

X Rule Y

10 65
30 45
50 25
70 5
I can't find the rule

Did you not notice that for every increase of 20 in the x's there is a decrease of 20 in the y's

so the slope is (45-65)/(30-10) = -1

so y = -x + b
using x=10, y = 65
65 = -10 + b
b = 75

y = -x + 75