JS challenge question

Question

I didn't understand this question. If I call the function guess(c,b,a); output is 21, that is understandable but in case of guess(a,c,b); output is 25, means c takes b's place. how? please help me to understand this.

var a=2,b=3,c=5;
function guess(c,b,a){
console.log(b*(c+a));
}
guess(c,b,a);
//output 21
guess(a,c,b);
//output 25