Find the missing length. Round to the nearest tenth, if necessary.
A. 441
B. 22.5
C. 64
D. 19.4

Using the Pythagorean theorem:
21^2 = √(8^2 + x^2)
441 = √( 64 + x)
441 - 64 = √x
377 = √x
x = √377
x = 19.4
The answer is D.