Answer: 4
Let's say line A and B are perpendicular lines.
If the slope of line A is -1/4, then the slope of line B is 4/1 = 4.
Note how the two slopes multiply to -1.
Your textbook might use the term "negative reciprocal" or "opposite reciprocal" when going from -1/4 to 4/1. We basically flip the fraction and flip the sign.
