This does not require a system of equations, that's a little overkill.
So, when you are setting up linear equations, you should know that a general linear equation looks like this: y=mx+b, where m is the slope and b is the y-intercept. So, you need to find the slope here, which you find using the equation
So, what points will you use for this little formula above?
The linear equation in 2 variables they are talking about is the above, as opposed to the x=4 or y=10 linear equations.
Once you find out the slope here, you can use the point-slope formula to find your equation
Ok, so you have your equation. Now just plus in 3 for the variable you used for miles and simplify.
Let me know if that helped