Atriangle has two sides of lengths 5 and 12. what value could he length of the third side be

I don't know if its a right triangle and we are trying to find the length of the hypotenuse
but if it is, use Pythagorean Theory
    c² = √a² + b²   
    c² = √12² + 5²
    c² = √144 + 25 
    c² = √169

            c=13

You would use the Pythagorean theorem which is A squared plus B squared equals C squared. The way you set it up would be 5 squared plus 12 squared equals C squared. Square the five and the 12 and then add Them and then take the square root of the solution which would be C squared and your answer would be 13