Use the points (1,3) and (2,1) to find the equation of an example line. The first number in the pair is the x coordinate the second number in the pair is the y coordinate. Insert both points of the line in the slope formula(m=(y2-y1)/(x2-x1)). Either y-coordinate can be y1 and y2, as long as the x-coordinates for the second part of the equation correspond. For example if y2 equals 3, then x2 must equal 1 in this example.

Insert the formula into a calculator (you can also solve the problem manually if you prefer). Subtract y1 from y2 (in our problem, solve 3 minus 1). Subtract x1 from x2 (In our problem, solve 1 minus 2). In this problem the solution is 2 divided by -1. When you divide the quantity in this problem you are left with -2. So the slope of the line equals -2.

Use the slope to find the y-intercept of a line. The y-intercept is represented by the letter b in the equation of a line. Solve for b using the equation y=mx+b. To find b, substitute the slope you found in the previous step (-2) for m. Then substitute one of the points on the line for y and x in the problem. We'll use the point (2,1). Now your problem is 1=-2x2+b.

Multiply -2 and 2, which equals -4. Now your problem is 1=-4+b.

Add -4 to both sides of the problem to get b alone. 1+-4 equals -3. So you're left with b=-3.

Substitute your solutions for m and b into the slope intercept equation (y=mx+b). This gives you y equals 2 multipled by x +-3. Now you can substitute any x point on the line and get the y intercept that corresponds to it.