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 + 20 ... and for 30 months, sub in 30 for m

c = 20 (30) + 20