The slope of a line perpendicular to another line is the opposite reciprocal of the slope of that line. Basically, the slopes of two perpendicular lines multiply to -1. The slope of this line is -1/5. To find the slope of a line perpendicular, we need to find out what number, multiplied by -1/5, would make -1. Or, we can take the opposite reciprocal (it's the same answer). This number is 5, so the slope of a line perpendicular to this line is 5.