Isn't the solution just 1+2+3+4+5+6 = 21?
What is your reasoning? First of all, what is your random variable $\displaystyle X $? Define that first. Then look at $\displaystyle E(X) $. There are $\displaystyle 6 $ outcomes. What is the probability of each outcome? You can also think about this as rolling a six-sided die.