Assume a linear relationship: c=ad+b. Insert given values: 3=3a+b; 4.80=6a+b. Subtract the first equation from the second: 1.80=3a, a=$0.60. Substitute a=0.60 in either equation: 3=1.80+b, b=$1.20. The required equation is c=0.60d+1.20, where c is in dollars and d in miles. The quantity b=$1.20 is a fixed charge.