There is an easier formula for the variance, derived from the one you have.
Recall two properties of the expectancy :
(where a is a real number)
(again, where a is a real number)
By (1), we have
Note that is a value, a number. It is not a variable anymore.
By (2), we have
(here, f(x)=x² and is the set of all possible values for X)