Classify the triangle based on the side lengths 7 15 and 21.

When given 3 triangle sides, to determine if the triangle is acute, right or obtuse:
1) Square all 3 sides.
2) Sum the squares of the 2 shortest sides.
3) Compare this sum to the square of the 3rd side.

if sum > 3rd side²   Acute Triangle
if sum = 3rd side²   Right Triangle
if sum < 3rd side²   Obtuse Triangle

Sides squared are:
49 225 and 441
Sum = 274
274 < 441 Therefore it is an obtuse triangle.

Source:
http://www.1728.org/triantest.htm

The answer will be C