The slope of the given line is m = 8
You can think of m = 8 as m = 8/1. Flip the fraction to get 1/8 and then flip the sign to get -1/8
Answer: -1/8
Note: Multiplying the original slope (8) and the perpendicular slope (-1/8) will lead to the result of -1. This applies to any two perpendicular lines as long as one of the lines isn't vertical.