The monthly cost (in dollars) of a long-distance phone plan is a linear function of the total calling time (in minutes). the monthly cost for 50 minutes of calls is $13.98 and the monthly cost for 78 minutes is $17.06 . what is the monthly cost for 77 minutes of calls?

(50,13.98)(78,17.06)
slope = (17.06 - 13.98) / (78 - 50) = 3.08 / 28 = 0.11

y = mx + b
slope(m) = 0.11
use either of ur points...(50,13.98)...x = 50 and y = 13.98
now sub into the formula and find b, the y int
13.98 = 0.11(50) + b
13.98 = 5.5 + b
13.98 - 5.5 = b
8.48 = b

so ur equation is : y = 0.11x + 8.48for 77 minutes...sub in 77 for x
y = 0.11(77) + 8.48
y = 8.47 + 8.48
y = 16.95 <=== this is the cost for 77 minutes