Answer:
sqrt 51
Explanation:
You should use the pythagorean theorem, which states a^2+b^2=c^2. So the missing side length for the smaller triangle is sqrt{2^2+3^2}=sqrt{4+9}=sqrt{13}. And the side length x is sqrt{8^2-sqrt{13}^2}=sqrt{64-13}=sqrt{51}