I’m assuming this these are the sides to a right triangle.
To find a missing side of a right triangle, you will use the Pythagorean theorem:
a^2 + b^2 = c^2
a and b are the legs and c is the hypotenuse.
The information given is not enough to solve this problem, but I will assume the hypotenuse is 15 and one of the legs is 9.
So:
9^2 + b^2 = 15^2
Calculate:
81 + b^2 = 225
Subtract 81 to both sides:
b^2 = 144
Square root both sides:
b = 12
The length of the third side is 12.
***This is only an assumption as to what the problem is. If it is Pythagorean theorem, check to see if the assumed sides are correct, if not, switch the numbers.
Hope this helped!