Xavier said the missing length is about 18.5 units. Without calculating, how can you tell that Xavier solved incorrectly? a right triangle with A as 21 and B as 28. trying to find the hypotnus.

See below.

Step-by-step explanation:

The hypotenuse must be longer than the the longest leg of a right triangle.  Since the legs are 21 and 28, the hypotenuse must be longer than 28.  His result is shorter than the shortest leg so it cannot be the hypotenuse.