The names PNM and MPN are perfectly valid. The order of the letters doesn't matter. In fact, there are 6 ways to name this plane if we just use the 3 letters. If we include "plane Z", then there are 7 total ways to name the plane.
For some reason, your teacher wants P to be the third letter as s/he mentioned MNP and NMP. But again, the order doesn't matter.
The two in the middle. B and C, I guess.
There is no square root of five...
Tan205° = .466 tan55° = 1.428 tan25° = .466 -tan25° = " so just tan25
