Match a point to a line

Here's a description of my problem. I have a set of 5 lines, y=f(x). Each line has a different slope. For a simple example: y1=4xI y2=5x y3=6x

I am given both the value of x, the value of y, and the equation of the 5 lines. My problem is matching the (x,y) value to the correct line. I want to say something like "If y(x) is on this line, then the slope is this value." I will then calculate additional parameters based on the slope. I feel like there is a simple command to make this happen, but I can't figure it out! Thanks.