what's the rule: 0 to 10 6 to 22 15 to 40?

Answer 1

(0,10)
(6,22)
(15,40)
slope between first two is (22-10)/6-0) = 2
slope between last two is (40-22)/(15-6) = 18/9 = 2
so the 3 points form a straight line.
equation:
y - 10 = 2(x-0)

y = 2x + 10