Answer:
length √17
Step-by-step explanation:
You must either share a diagram showing that this is a right triangle, or say in so many words that it's a right triangle.
We'll apply the Pythagorean Theorem. First, let's square 8 and 15, obtaining 64 and 225. Next, notice that the sum of 64 and 225 is 289, which is the square of 17. The Pythagorean Theorem states in this case that 8^2 + 15^2 = 17^2.
The third side is the hypotenuse of this right triangle. That hypotenuse / third side has length √17. This can't be reduced / simplified.
