Answer:
A negative slope and a positive slope with the same absolute value (magnitude) are indeed perpendicular to each other. In other words, if you have slopes of -3 and 3 (or any other pair of slopes where one is the negative of the other), they are perpendicular.
In general, two lines are perpendicular if the product of their slopes is equal to -1. So, in your example, (-3) * (3) = -9, which is indeed equal to -1. This means the lines represented by these slopes are perpendicular.
Explanation: