Q. # 21. find the missing side length

The missing length is the hypotenuse. 
Using the Pythagoras theorem,  
 c²=a²+b²

c²=8²+6²
    =64+36
     =100

c=√100
   10

To solve this problem you musta pply the proccedure shown below:
 1. You must use the Pythagorean Theorem, which is:
 a^2=b^2+c^2
 Where a is the hypotenuse and b and c are the other sides of the right triangle.
 2. Therefore, you have that the hypotenuse is:
 a=√(b^2+c^2)
 a=√(8^2+6^2)
 a=10
 The answer is 10