Geometry Homework Help

I need to know find the distance between two parallel lines, and I know how to do most of it. So far, I have the y=mx+b equations of both parallel lines, but I don’t know what two points from the lines to select to find the distance between. In other words, I don’t know what two points to plug into the distance formula. I do not have a graph and I am not allowed to use/make one. How do I determine algebraicly (I don’t know how to spell it sorry lol) which points to use from each line to determine the distance between two parallel lines?