P?MDAS
PEMDAS stands for the order of operation:

P = PARENTHESES.  Do any operations undone in innermost parentheses first.

E = EXPONENTS. After taking care of all inner-most parentheses, do any 
exponents.

MD = MULTIPLICATION/DIVISION.  After taking care of all exponents, do
any multiplications and divisions in order as you come to them going left to
right.

AS = ADDITION/SUBTRACTION.  After taking care of all multiplications and
divisions in order as you come to them going from left to right, the additions
and subtractions in orde left to right.

So the missing letter is E, for "Exponents" 

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ROYGB?V
ROYGBIV, (think of a man whose name is "Roy. G. Biv") stands for the order of
colors you see in the spectrum (such as in the rainbow) left to right are Red,
Orange, Yellow, Green, Blue, Indigo, and Violet  

So the missing letter is I, for "Indigo", which is actually purple.

Edwin