To start, we know the basic equation from what us given:
![c = 25 + mx](https://tex.z-dn.net/?f=c%20%3D%2025%20%2B%20mx)
And we want to find m. Fortunately, we have some numbers to plug in to find m. We can make an equation like this
![67 = (210)m + 25](https://tex.z-dn.net/?f=67%20%3D%20%28210%29m%20%2B%2025)
by taking the amount charged and the amount of miles driven to get that charge given in the original problem.
Now we solve.
![67 - 25 = 210m + 25 - 25 \\ 42 = 210m \\ \frac{42}{210} = \frac{210}{210} m \\ \frac{1}{5} = m](https://tex.z-dn.net/?f=67%20-%2025%20%3D%20210m%20%2B%2025%20-%2025%20%5C%5C%2042%20%3D%20210m%20%5C%5C%20%20%5Cfrac%7B42%7D%7B210%7D%20%20%3D%20%20%5Cfrac%7B210%7D%7B210%7D%20m%20%5C%5C%20%20%5Cfrac%7B1%7D%7B5%7D%20%20%3D%20m)
Now we can put that in our equation to finish it.
![c = \frac{1}{5} x + 25](https://tex.z-dn.net/?f=c%20%3D%20%20%5Cfrac%7B1%7D%7B5%7D%20x%20%2B%2025)
And that is the final equation