You need to simplify the double negative to a positive and then add the absolute values of of the numbers, keeping the sign of the larger number.
First step: Simplify the double negative. The double negative can be simplified to a positive. So, rewrite the expression as (-6) + 15
Second step: You have to add (-6) + 15. To add (-6) + 15, we have to add the absolute values of the number and keep the sign of the larger number. The absolute value of (-6) is 6 and absolute value of 15 is 15. Since 15 is larger, you keep the sign. So, (-6) + 15 equals 9
Therefore, the answer is 9.