Simplify sum k = 0 to n of k^2 (n choose k).

1)
Simplify sum k = 0 to n of k^2 (n choose k).
2)
Find an explicit formula for a_n so that a_0 = 1, a_1 = 2, and a_{n+2} = 5a_{n+1}-3a_n.
3)
Find an explicit formula for the number of tilings of a 2xn grid with 1×2 and 2×1 and 2×2 tiles.
4)
Find all Fibonacci numbers that are divisible by 3.
Process and explanation required. Thanks