How did he do that? I follow a regular "do the integral inside first" then "do the integral outside second" after I have initially just put the '3' completely outside but he seems to get the 3 outside first in the form of 6 and then shrinking the whole of it. (???) I would normally be OK with not getting it but in this case he gets a result of 1/4 and I get 1/12 with my method. The freakiest thing? I saw a postgraduate mathematician getting 1/12 as well. How did the Professor get 1/4? The question did have a prerequisite of finding fx(x) (the above is from fx,y(x,y)), but I doubt it's related..