I have two questions about finding the slope of a line. First question: Is the formula Y2-Y1 over X2-X1? Second question: When I am given a line on a graph, how do I find Y1, Y2, X1, X2 to use in finding the slope?

yes on the first question.

On the second question, pick any two points on the line, label one of them y2, and one of them y1. It matters not in what order.