Let M,N be subspaces of Rn. The perp operator acts on subspaces, we can ask how it

Question:

Let M,N be subspaces of Rn. The perp operator acts on subspaces, we can ask how it interacts with other such operations.
(a) Show that two perps cancel: (M┴)┴ = M.
(b) Prove that M N implies that N┴ ⊂ M┴.
(c) Show that (M+ N)┴ = M┴ ∩ N?.