You can
put this solution on YOUR website!i have never come across this terminolgy before - i guess it is an Americanism.
Anyway, looking on the web, it is as follows:
Start with the x-axis as the "initial side". Then turn to point at the coordinate (here, (12,-16)). The line you end up on (the hypotenuse in this case) is the terminal side.
So, sketching axes. Plot the point (12, -16) in the 4th quadrant. Start at the origin, pointing along the +ve x-direction and then turn to the point to create the hypotenuse..this is your angle

, and then

= opp/adj which gives
tan

= -16/12
tan

= -4/3
jon.