We have the following:
One of the ways to know what happens between two lines is to know the slope of these lines.
That is, by knowing the slope of both lines we can know the behavior between them.
We have that when two slopes are equal the lines are parallel and when the slopes are opposite the lines are perpendicular.
Thus:
1.How can you know if two lines are parallel?
They have the same slope
2. How can you know if two lines are perpendicular?
The slopes are Opposite and Reciprocals of each other
3. How can you know if two lines are neither parallel or perpendicular?
The lines cross, but not at a 90 degree angle