To solve this problem, we must keep in mind that adding a negative number and subtracting a positive number are the same thing (both include a positive and a negative sign). Using this knowledge, we can begin to simplify the above expression:
x + (-8y) + (-5) + 15y
x - 8y - 5 + 15y
Now, we must combine like terms. This means that we must add/subtract the constant terms and the terms of the same variable. Because there are two terms with the variable y, we should add those together to simplify the expression.
x - 8y - 5 + 15y
x + 7y - 5
Therefore, your answer is x + 7y - 5 (keep in mind that the terms can be rearranged into any order as long as you keep the signs so that the terms have the same value).
Hope this helps!