The summation binds $x$ to a (series of) particular value(s). In your first line that ends with 1/2, you're using the union of all values $\geq 2$, but that's wrong for the probability expression in the sum. You have to fill in 2, 3, 4 for $x$, one by one for both $x$'s in the sum.

(I'll let you work out the rest.)

The summation binds $x$ to a (series of) particular value(s). In your first line that ends with 1/2, you're using the union of all values $\geq 2$, but that's wrong for the probability expression in the sum. You have to fill in 2, 3, 4 for $x$, one by one for both $x$'s in the sum.

(I'll let you work out the rest.)

[Not sure how you got 8, but it doesn't matter!]