Answer:The answer is -4.
Step-by-step explanation: Perpendicular lines are lines that meet at a certain point; both making a 90 degrees angle. Which means that in the context of this situation the other line will have the exact same slope only pointing a different way. However the line pointing a different way has a crucial point to it since it changes the values to the slope . For example a negative slope is going down and a slope that is positive is going upwards. So the only logical answer here would be -4.
Hope this helps!
(P.S. Image posted below if i confused you too much with the explanation.)