Two sides of an isosceles triangle have lengths of 10 inches and 21 inches, respectively. find the length of the third side.

That will depend on if 21 is the hypotenuse (longest side in the triangle) or not. if 21 is the hypotenuse, you would use the pathagorean theorem (a*+b*=c*)
* = squared

you would set it up as 10*+b*=21* and solve. if 21 is another leg of the triangle and not the hypotenuse, then you would then substitute b out with 21 and leave c as c and then solve.

The answer is 21 because in a isosceles triangle 2 sides have to be the same.