I only did the first, maybe this will give you a hint how to proceed.
Let the angle between the positive x-axis and y=m1x be called
and let the angle between y=mx and the positive x-axis be called
Then the angle is given by
How do you figure out ?
It has something to do with Tangens.
Letīs say you have the line
Then where the index 1 and 2 stand for the different lines.
Then you do the same for the other line.
And you end up with: