[MATH HELP] how to find fixed cost?

The second passenger went an extra 13km, and it cost him an extra
37.10 - 21.50 = 15.60

That means each km cost him
15.60 / 13 = 1.20

So: the first passenger's trip cost him
total cost = fixed cost + variable cost
total cost = fixed cost + (number of kms * cost per km)
21.50 = fixed cost + (15 * 1.20)
21.50 = fixed cost + $18.00
3.50 = fixed cost

Done :)

F:Fixed Cost | X:rate per KM

F+15X=$21.50 (15KM Trip) --1
F+28X=$37.10 (28KM Trip) --2

Equation 2 - Equation 1
F will be eliminated, 13X=$15.60, therefore X =$1.20

F+15(1.2)=21.5
F=21.5-15(1.2)
F=21.5-18
3.50

Therefore fixed cost is $3.50 and $1.20 is rate per KM.

Points given are (15,21.5) and (28,37.1)
Slope = 15.6/13 = 1.2
y-21.5 = 1.2(x-15)
y = 1.2x + 3.5
Fixed cost = $3.50
km charge = $1.20 per km

Set up a series of equations
First one is 15x + y = 21.5
Second is 28x + y = 37.1

After using substitution of the first into the second equation, you get x = 1.2
Plug that into either of the 2 equations and y =3.5, which is the fixed cost