The tan function goes by the syntax (setup) of tan(O/A) where O is the side opposite of the chosen angle, and A is the adjacent (next to) side from the angle.
To find an angle, you would use tan^-1(O/A);. This is called the tangent inverse.
This gives you tan^-1(12/16) which is 36.87 (if you need to find an angle.
If you just want to find tan(p) you would do tan(12/16) which is 0.01
:)