More precise expression of the answer So the problem says "Let $X$ be the sum of outcomes of rolling $2$ dice, where the outcome for each dice appears with equal probability. What is the pmf of $X$?"
I got: $X$ could take on any value in the set $\{2,3,4,5,6,7,8,9,10,11,12\}$
The probability mass function is
$$f(x) = \begin{cases}\frac1 {36} & \text{ if }  x∈{2,12}\\\frac1 {18} & \text{ if }  x∈{3,11}\\ \frac1 {12} & \text{ if }  x∈{4,10}\\ \frac1 {9} & \text{ if }  x∈{5,9}\\\frac5 {36} & \text{ if }  x∈{6,8}\\\frac1 {6} & \text{ if }  x∈{7}\\ \ 0 & \text{ if }  other\end{cases}$$ 
Is the answer correct? If so, is there any way I can write the answer in a more precise way instead of listing all the values of $x$?

Your answer is correct and precise; it may not be concise.
For a more concise answer (which is not always possible, but is possible in this case) try to rewrite each fraction so it has the same denominator $36$, and try to see if there's a formula tying the numerator to the result(s) with that probability!
