(3,9)(5,15)
slope = (15 - 9) / (5 - 3) = 6/2 = 3
y = mx + b
slope(m) = 3
(3,9)...x = 3 and y = 9
sub and find b, the y int
9 = 3(3) + b
9 = 9 + b
9 - 9 = b
0 = b
so ur equation is : y = 3x + 0....can be written as y = 3x.
after 12 months...
y = 3x...x = 12
y = 3(12)
y = 36 inches