You call the independent variable any name you want and the dependent f(that variable you defined earlier). Meaning the output of that f(variable chosen) is the output that the function based on that variable you chosen gives you. 

In order to determine the slope, which represents the rate of change in a linear equation word problem, you need to identify two variables that are changing in relation to each other. For example, in a problem involving distance and time, the distance traveled would be one variable and the time it takes to travel that distance would be the other variable.

To determine the rate, you would divide the change in one variable by the change in the other variable. For instance, if you are given that a car travels 60 miles in 2 hours, you can find the rate of the car by dividing the distance traveled (60 miles) by the time taken (2 hours), which gives you a rate of 30 miles per hour.

It's important to pay attention to the wording of the problem and identify which variables are related and how they are related in order to determine which one should be divided by the other to find the rate.