Ineed with trigonometry. these are about angles if you could anser one or both thats greatly appreciated

20)   

check the picture below, keep in mind that negative angles move about clockwise, same way a clock goes.

21)



now, the little bit more makes that angle, if we nevermind the first revolution, one could say that angle started really from 0, though we know it didn't, however if that angle did so, is also a co-terminal angle of 17π/6.

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table