21 - (-18) + 15 - 126

To solve this expression, we can follow the order of operations (PEMDAS/BODMAS).

First, we need to simplify the expression inside the parentheses:
21 - (-18) + 15 - 126
= 21 + 18 + 15 - 126

Next, we can add the numbers together:
= 39 + 15 - 126

Finally, we can subtract the numbers:
= 54 - 126
= -72

So, 21 - (-18) + 15 - 126 equals -72.