I made this more complicated than it should have been. I did the area of the rectangle HR = 1/2(HR) + xy + 1/2(x)(H-y) + 1/2(y)(R-x) then rearranged for y. Also tried pythagoras and i was like wtf so i stopped.
With question 11d two of the values of x are pi/2 and 3pi/2, but tan(pi/2) and tan(3pi/2) are undefined. Would it be wrong to say that x can't equal pi/2 and 3pi/2? A similar question showed up in the 2014 2U multiple choice (question 7) and they said that because it's undefined it can't be...