Hi,
c(x) is a linear function, if you graph it, you obtain a straight line.
The slope is the coefficient of x, therefore the slope is 2.
When the ride starts (x = 0), the start cost is 4 dollars (or whatever the currency is), then every minute spent on the moving taxi, as many as 2 dollars need to be paid to taxi driver.
I hope these help.
Green eyes.
