Rule:  

Use the Pythagorean theorem to find the length of the
hypotenuse of a right triangle whose legs are the shorter two
sides.

a. If the longest side is shorter than this hypotenuse, the
triangle is an acute triangle.
b. If the longest side is longer than this hypotenuse, the
triangle is an obtuse triangle. 
q. If the longest side is equal to this hypotenuse, the
triangle is a right triangle.

c² = a² + b²
c² = 20² + 21²
c² = 400 + 441
c² = 841
 c = 29

The longest side is shorter than that hypotenuse, so the triangle
is acute.

Edwin