Answer: 1/3
Explanation: To most students, perpendicular lines are simply lines that meet at a 90° angle.
However, when doing problems like these, it's important
to think about perpendicular lines in terms of their slopes.
As a rule, the slopes of two perpendicular lines
are always negative reciprocals.
What that means is that the signs will be opposite
and the fractions will be flipped.
So we can think of the green line having a slope of -3/1.
If we flip it and change the signs, that is what our slope
will be the the red line.
So flipping and changing the sign we have 1/3.
So the slope of the red line is 1/3.