Alright. I'm not the best with these problems, but let me give it a shot,
First, let's use the Law of Cosines. According to the law, we have
\(BC^2 = AB^2 + AC^2 - 2(AB* AC) cos ( BAC)\)
Now, we know a lot of the information given already, so we want to find cos BAC. Thus, we have
\(8^2=3^2+7^2-2(3*7) \cdot cos(BAC)\)
Now, isolating BAC, we have that
\(64=16 \cdot cos(BAC)\\ cos(BAC)=4\)
Now, why not apply Law of Cosines again? Let's do it! :)
This time, let's use the equation
\(DE^2 = AE^2 + AD^2 - 2(AE * AD) cos (BAC) \)
We figured out and know every term in the right side of the equation, so plugging in all the information, we have
\(DE^2 = 9^2+21^2-2(9*21)*4\)
mmm...let's check something out.
This simplifies to \(DE^2=-990 \), which defintely IS NOT possible.
Now, disclaimer though....it's kinda late at night, and my brain is fried. my solution is probably faulty.
...