Answer:
![y(x)=100+0.1x](https://img.qammunity.org/2021/formulas/mathematics/college/rxiwfb9muhkv4xb8iooe411180oj6wp9zz.png)
Explanation:
Let y represent the total fee (in dollars) of a trip where they climbed x vertical meters.
We know that there is an initial fee of $100, so we know that if we climb x=0 meters, we have a fee of y(0)=100.
![y(0)=100](https://img.qammunity.org/2021/formulas/mathematics/college/semlbyzsz4n9d1koriiqgfgxzb8dw8e8kv.png)
As there is a constant fee (lets called it m) for each vertical meter climbed, we have a linear relationship as:
![y(x)-y(0)=m(x-0)\\\\\\y(x)-100=mx\\\\\\y(x)=100+mx](https://img.qammunity.org/2021/formulas/mathematics/college/jil71vsefcnp16jzfrppqmevt66m1jfnli.png)
We know that for x=3000, we have a fee of $400, so if we replace this in the linear equation, we have:
![y(3000)=100+m(3000)=400\\\\\\100+3000m=400\\\\3000m=400-100=300\\\\m=300/3000=0.1](https://img.qammunity.org/2021/formulas/mathematics/college/1a8dvlb039samzpi3q3xim32iy9sdajmsf.png)
Then, we have the equation for the total fee in function of the vertical distance:
![y(x)=100+0.1x](https://img.qammunity.org/2021/formulas/mathematics/college/rxiwfb9muhkv4xb8iooe411180oj6wp9zz.png)