why can't the answer be 6.0 ?

Answer 1

Because it returns ant Int (integer). That means numbers like : 1 or 10 or 0 or 66. Numers with no decimals. If you wanna return a decimal number, you would use a float or a double

Answer 2

What do you mean?

Answer 3

since the data type specified is double so the answer can be 6.0