3D Trig Help 2 (1 Viewer)

[Andy005]

https://drive.google.com/open?id=0B_RlXPQTW1cRREVqWmZRRHU1LTg

For a) I used the angle sum of a polygon formula, would this be the correct approach or would I need to do so by other means?

For b) I'm unsure on what to do but I'm probably overthinking it.

Thanks for the help.