A 3-mi cab ride cost $3.00. A 6-mi cab ride cost $4.80. Find a linear equation that models cost c as a function of distance d.

The difference in cost for 3 miles versus 6 miles is $1.80, so we can infer that the cost in dollars (c) is 0.6d plus a fixed charge (x). We can substitute d=3 to find x like so:

3 = 0.6 (3) + x

3 = 1.8 + x

Therefore x = 1.2

Therefore c = 0.6d + 1.2