To solve this expression, we need to follow the order of operations, which is parentheses, exponents, multiplication and division (from left to right), and addition and subtraction (from left to right).

Let's solve it step by step:

1. First, we have the expression inside the parentheses: (8 + 21 - X). We can't simplify it further without knowing the value of X.

2. Next, we have the multiplication: 8 x 13 = 104.

3. Now, we can add the numbers: 7 + 13 + 104 = 124.

4. Finally, we divide the sum by the expression inside the parentheses: 124 : (8 + 21 - X).

So, the simplified expression is 124 : (8 + 21 - X).