(7,82)(11,122)
slope = (122-82)/(11-7) = 40/4 = 10
y = mx + b
slope(m) = 10
(7,82)...x = 7 and y = 82
now we sub and solve for b, the y int
82 = 10(7) + b
82 = 70 + b
82 - 70 = b
12 = b
so ur equation is : y = 10x + 12
when x = 4
y = 10(4) + 12
y = 40 + 12
y = 52 <== total cost is 52 for 4 lessons
