It says: sketch a resonable graph of y=f(x), if it is known that f(2)=80, f(5)=40 and f(10)=25. I understand that, what I don't understand is how to find the value of f(7) or when it says to find the difference in x values between the points where f(x)=70 and where f(x)=30?

Once you've sketched your graph you can estimate those values directly from the graph. To get f(7) you start on the horizontal axis at 7, and go straight up until you hit the graph, then over to the vertical axis. The value you hit on the vertical axis is your estimate of f(7).
To find where f(x) = 70 you start at 70 on the vertical axis, go straight over to the graph, then down to the x axis. To find where f(x) = 30 you start at 30 on the vertical axis, go straight over to the graph, then down to the x axis. The distance between the points you meet on the x axis is the requested difference in x values.