Yes, you are right. Step 15 is about solving and graphing a system of 2 linear equations, where the 2 equations are written in the code. The code shows how to use np.linspace to generate a series of values for x, then calculate the corresponding values of y and plot them. Step 22 does similar thing, but uses for loop instead and displays a table of values.
So now your problem is how to get any equation of y in terms of x from input and calculate the y values from different values of x. Well, this step is not demonstrated exactly in the teaching materials. There are demonstrations showing getting input of any equation in terms of x and solve for x. To tackle this task, you may consult the documentation of SymPy, in this section there are discussions about how to convert strings into SymPy expressions and evaluate the values for different values of x.