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

The slsope of the graph of the line = 4.8 - 3 / 6-3 = 0.6

so we can write the equation as

c = 0.6d + b where b is some constant

When d = 3 c = 3 so we have

3 = 0.6 * 3 + b

giving b = 3 - 1.8 = 1.2

Therefore the required linear equation is

c = 0.6d + 1.2