A3-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