Is a triangle with measures of 6 ft, 21 ft, 23 ft a right triangle

To see if this possible triangle is a right triangle we have to apply  the Pythagorean theorem: 
In any right triangle:
hypotenuse²=leg₁²+leg₂²
In this case the possible hypotenuse will be the largest side:
hypotenuse=23 ft
and the sides the legs:
leg₁=6 ft
leg₂=21 ft

Therefore:
hypotenuse=(23 ft)²=529 ft²
leg₁²+leg₂²=(6 ft)²+(21 ft)²=477 ft²

becuse: 
hypotenuse²≠leg₁²+leg₂²
529 ft²≠477 ft²
This possible triangle is not a right triangle.