To solve this problem, you can plug your values for the triangle's side lengths into the Pythagorean Theorem. The Pythagorean Theorem is a² + b² = c², where c is the hypotenuse of a right triangle. The hypotenuse is the longest side of the triangle, so when you plug in your values for this triangle, you must plug 23 for c, like so:
6² + 21² = 23²
Now you can solve; if 6² + 21² is indeed equal to 23², the triangle is a right triangle.
6² + 21² = 23²
36 + 441 = 529
477 ≠ 529
477 is not equal to 529, so this triangle is not a right triangle.