You're right for A) and C), except for C) I'd probably use |B - A| = |B|-|A intersect B|

For D)

Note that |B - C| = |B| - |B intersect C|

=> 10 = 21 - |B intersect C|

=> |B intersect C| = 11

also note |C - B| = |C| - |B intersect C|

=> 12 = |C| - 11

=> |C| = 23

Finally,

|B U C| = |B| + |C| - |B intersect C|

=> |B U C| = 21 + 23 - 11

=> |B U C| = 33

I'm a little rusty on this, so there's probably a shorter way to do this, fiddle around with it some more and see