dicrete mathematics recurrence relation and binomial expansion HELP!!!

1. what is the sequence Xn for n>= 0 is given by recurrence relation X0 = 0, X1 = 3,

Xn+2 =Xn+1 +2Xn, n>=0.

a) find X4.

b) find a formula for Xn in terms of n.

2. Give the x3 Y4 term of the followwing binomial expansion:

a)(x-y)7.

B(x+3y)7.

3.finally , five objects are chosen from nine objects (as usual, the objects are distinguishable,

and there is no replacement).

a) how many ways can this be done, if the order of choice matters?

b) how many ways can this be done, if the order of choice does not matter?

can some one plz help me with these questions i would be much greatful