The answer using pythagorean theorem

12.32 is the answer.

Step-by-step explanation: Using the pythagorean thereom we can solve this by using the formula A^2+B^2=C^2.

We substitute in the values,

17^2+B^2=21^2.

Now we simplify.

289+B^2=441

We subtract the 289 from both sides,

B^2=152                                                                                                                                                            

Now we use square root to find the answer.

12.32 is the answer.

Hope this helps!