Try the Pythagorean theorem, [tex] a^{2} + b^{2}= c^{2} [/tex]. Does [tex] 6^{2} + 21^{2} = 23^{2} [/tex]? If it does, you have yourself a right triangle. If it doesn't, then it isn't a right triangle.
According to the pythagorean theorem a^2+b^2=c^2 where c is the largest side. The square root of 6^2+21^2=21.84 which does not equal 23. So it is not a right angle triangle.
