At line 6 you are declaring an array with 0 elements and assigning 3 to it.That part is wrong.
The general syntax for declaring an array is-
1 2
int a[5] = {2,1,3,5,4};
int b[5] = { 0 }; //initialise all to 0
Please refer to the c++ tutorial at this site or your book.
And your implementation looks a little quirky .here's a hint .create a function "int u(int n)" and if the valve of n is equal to 0 return 0 else return 3*u(n-1)+4