If you divide $6.00 by 120, you come up with 5 cents per mile. So the slope of the line (m) would be 0.05, or 1/20.
We also know that it costs $40.00 at 100 miles, so when x=100, y=40. If you multiply 100 by 0.05 you get 5, which means the rate goes up $5.00 for every 100 miles. So if you take your original y=40 ($40.00 for 100 miles) and subtract 5, that means y=35 for 0 miles, and that gives you your zero-intercept (b).
So your equation would be y = 1/20x + 35
Now, if x = 300 miles, then
y= 1/20*300 + 35
y = 15 + 35
y = 50, therefore it would cost $50.00 to drive the rental car 300 miles.
