Probability Distribution

X and Y are 2 independent random variables as follows:
x P(X = x) y P(Y = y)
1 1/6 1 a
2 1/6 2 (3/5) - a
3 1/6 3 1/10
4 1/6 4 1/10
5 1/6 5 1/5
6 1/6
(i) Let E be the event { X - Y ≥ 4}, find P(E) in terms of a.
(ii) Let F be the event { X + Y = 6} and E and F are independent, find the value of a.