Use xyLine to type proper equations?

Sigh. Okay. From simpleFunctions vs. xyLine vs. parseEquation
the only one I really understood was xyLine. I just want to make a slide where students enter the equation of a proportional relationship. I can’t understand the code enough from the other examples to do this , but the `correct: myLine.yIntercept=0’ made sense to me.
I don’t suppose parse equation code is easy enough to give me an example for a correct input of the form y=kx where k is any rational number?