Question 713507
Assign those numbers to an arrangement of (x,y) coordinates; TWO of them.


Do as x for miles and y for cost.  The points are (100, 50) and (400, 68).  Two points - Find the linear equation.


Slope, m=(68-50)/(400-100)
m=18/300, {{{m=3/50}}}


Point-Slope Form For a Line: {{{y-y[0]=m(x-x[0])}}}.  Substitute for m, Pick either of the data points for ({{{x[0]}}},{{{y[0]}}}).  Simplify.