7+21:(15-8)=

Let's solve the expression step by step:

1. Start with the expression: \( 7 + 21 : (15 - 8) \)
2. Calculate the parenthesis: \( 15 - 8 = 7 \)
3. Now the expression is: \( 7 + 21 : 7 \)
4. The division comes next: \( 21 : 7 = 3 \)
5. Now, substitute back into the expression: \( 7 + 3 \)
6. Finally, add: \( 7 + 3 = 10 \)

So, the result of the expression \( 7 + 21 : (15 - 8) \) is \( 10 \).