Why use a formula? Draw a sketch! Call P = (249, b) the point where the vertical line meets the second line. (The y-value is unimportant because you are just making a sketch.) Draw a horizontal line one unit long starting at P. (That is, join (249, b) to (250, b).) Next, draw a vertical line from (250, b) to the slanted line you were given, and from that point draw a horizontal line one unit back to the given vertical line. Mark the angle at P that you are interested in. You now have a right triangle and you know the sides opposite to and adjacent to your angle. That means you know the tangent of your angle.