To find perpendicular slope you basically take the slope of the given line and reverse whatever sign it had previously. (If it was negative, it would become positive. If it was positive it would become negative.)
By doing -A/B it make the previous slope negative.
Brandon only flipped the fraction which would just result in the slope being slightly steeper, or in this case, less steep than the previous line, rather than making it perpendicular.
Hope this helps! Stay safe and if you have any questions feel free to ask!
