y=3x-7

I'm not really a mathematical expert, but I don't know how you'd be able to work the equation when there's 2 unknown numbers. There's limitless options.

y = 3x - 7

This is an equation for a line. A line has an infinite number of points on it. For example one point would be (0,-7) where an x-value of 0 produces a y-value of -7. Points along this line can easily be obtained by substituting values of x and solving to get the y-value or vice versa.