Modular Arithmetic order of operations In an assignment, I am given $E_K(M) = M + K \pmod {26}$. This formula is to be applied twice in a formal proof, so that we have $E_a(E_b(M)) =\ ...$. What I'm wondering is; is the original given formula equal to $(M + K)\pmod{26}$, or $M + (K \mod{26})$? This will obviously make a big difference down the line.
I do suspect the first ($(M + K)\pmod{26}$), however I want to be certain before I move forward in my proof.
NB: I did not tag this as homework as this is not actually part of the problem, rather just a clarification.

Without further context it is impossible to determine what is intended, because there are many abuses of the mod notation.
However, with high probability, the expression $\rm\: M\!+\!K\ (mod\ 26)\:$ denotes the remainder of $\rm\:M\!+\!K\:$ when divided by $26\,$ (or the entire residue class $\rm\ M\!+\!K+26\,\Bbb Z\:$ containing the remainder).
Your other possible interpretation, $\rm\:M + (K\ mod\ 26),\:$  is much more rare, though it does sometimes occur when discussing parity, e.g. authors may write $\rm\:n\ (mod\ 2)\:$ or $\rm\: n\ mod\ 2\:$ in arithmetical expressions that depend upon the parity of $\rm\:n.$ 
In the first case one is performing modular arithmetic, i.e. addition modulo $26$. In the second case one is performing integer arithmetic, i.e. normal integer addition. If the ambient context reveals which type of arithmetic is intended, then you can infer the meaning of the notation.
