Sorry but I'm only permitted to answer your first question. Please post each of the others as individual questions and then they will be answered promptly.
A is at point (5;0) which means it is on the x-axis. B is at (7;3).
Draw a triangle along a set of axes marking A at (5;0). Then at (7;0) on the same axis plot a point - call it C if you like. From C draw a vertical line up until you reach (7;3) and mark point B. Now join A and B. Your 90 degree angle is at point C.
The distance from A to C is 2 units (7 - 5) and the distance from C to B is 3 units (from x-axis to 3).
Therefore tan theta = 3/2.
Use the shift key or 2nd function key on your calculator and you will get
tan theta = 56,3 degrees.