var merge = function(array1, array2, callback){
  //your code here.
  }

var x = merge([1, 2, 3, 4], [5, 6, 7, 8], function(a, b){
  return a + b;
});

//x should now equal [6, 8, 10, 12].

var merge = function(array1, array2, callback){
  for(var i = 0; i < array1.length; i++) {
      callback(array1[i], array2[i]);
  }
}

var x = merge([1, 2, 3, 4], [5, 6, 7, 8], function(a, b){
  return a + b;
});