Oh!
Do you use apple computers/devices?
You can take a screenshot, go into the photos app, and press the "edit" button.
Then, go to the three dots at the top, and press "markup". From there, you can create text boxes or draw the variables.
But, if you are using windows, I am unsure of how to help... I could just do it for you, but guest already did that.
Maybe you could learn Asympote! It's kind of hard at first, but once you get the hang of it, it would be really easy to add x, y and z to the lines.