Answer 1

On first call
f(a, 3)
// *a = 12 and *a%2==0 true. So
=> 12 + f(a+1, 2)
// *a now is 7, and *a%2==0 false.
=> 12 + 7 - f(a+1, 1)
// now *a is 5 and *a%2==0 false.

=> 12 + 7 - 5 - f(a+1, 0) //now *a out of blounds,

=> 12 + 7 - 5 - 0 = 14

Answer 2

It's output is
12 + 7 - 5
=14

*a points to first element of array..
On even number calling *a + f(a+1, n-1)
On odd number calling *a - f(a+1, n-1)
Until n>0.