You are just asked to setup a function. What you need to do is to write down:
y(x) = "fixcost" + "cost per mile" * x. Then you need to solve this for x = 350. For the second question set y = 150 and solve for x.
The cost of hiring a car for a day is £50 plus £0.55 per mile travelled. If £y is the cost of hiring a car for a day and x is the number of miles travelled write down the relationship between x and y. Draw the line represented by this equation, identifying the gradient and intercept. If the car is driven 350 in a day, what is the cost of hire? How far can I travel for £150?
As far as I know you can plot such functions in Excel without giving values. There are many programs however, that do the same job. If you use a Mac e.g. Grapher is installed on your system. For Windows there exists plenty of freeware.
If you want to draw it on paper, however: This function is linear. That means that it will be drawn as a line since the relative change from x to y remains the same over all values of the function. Calculate two random values (what you already did) and draw a line that connects both points. This will be your graph of the function.