Question 136939
You have two payment options
{{{c = 42 + 0.14m}}} and {{{c = 84 + 0.07m}}}

You are asked to find out at which point the two options cost the same. 

{{{42 + 0.14m = 84 + 0.07m}}}
{{{0.07m = 42}}}
{{{m = 600}}}
So if you go over 600 miles, the option where miles cost 7 cents apeice, will result in a lower total cost.

Graphically, the line that is 'above' the other line at any specific mileage is the more costly option

{{{graph(600,400,-100,900, -100, 200, 0.14x+42, 0.07x+84)}}}