addTwoNumbers accepts 2 arguments (x and y).
add5ToNumber is assigned to the output of calling addTwoNumbers with only 1 argument, which results in another function that "saves" the first argument (x -> 5) and accepts one other argument (y).
When you pass 6 into add5ToNumber, its passing the saved x (5) and the given y (6) into addTwoNumbers, resulting in 11
