rotating lines in 2d space
I'm struggeling with with a problem and culd use some help.
I have 2 lines somewhere in 2D space. One line is described by two known points p1, p2.
The other line is described by the unknown points p3, p4, but with a known length.
Now I need to connect p3 of the second line to either p1 or p2. The direction/alignment of the second line is defined by a known angle between the first line.
My problem now is to determine the second point p4 of the second line, given by p1, p2, p3 connected with either p1 or p2 and the angle between those lines.
I guess I have to apply some sort of rotation but I can't figure out how.
Thanks in advance for any help!