Given the following 2 functions - what is the value of calling fb(4,2)?

function fa(x, y){

if (y == 0) return 0;

return (x + fa(x, y-1));

}

function fb(x, y) {

if (y == 0) return 1;

return fa(x, fb(x, y-1));

}

please explain the answer so I can apply the concept to other questions. Thank you.