Lines are perpendicular to one another if they intersect at a given point. These lines also have opposite slopes. If line A has a slope of 5, then to find a line perpendicular, you would flip the 5 to 1/5. The sign of the slope would also change so it this example a line perpendicular to line A would have a slope of -1/5. The y-intercept does not matter. They can share it or it could be different.