The cost, c, of a health club membership is related linearly to its length in months, m. if a 12-month membership costs $260 and an 18-month membership costs $380, which equation can be used to find the cost of a 30-month membership?

(12,260)(18,380)
slope = (380 - 260) / (18 - 12) = 120/6 = 20

y = mx + b
slope(m) = 20
(12,260)...x = 12 and y = 260
now we sub and find b, the y int
260 = 20(12) + b
260 = 240 + b
260 - 240 = b
20 = b

ur equation for the line is y = 20x + 20

the cost of a 30 month membership..
y = 20(30) + 20
y = 600 + 20
y = 620

so basically, ur equation (with ur variables) is :
c = 20m + 20and for 30 months, sub in 30 for m
c = 20(30) + 20