Well, I have a couple of questions. What is your definition of "scalene" triangle? Have you already proved that "if a triangle is isosceles then it is not scalene"?
The language could be a little more explicit, but looks clear enough to me to get the job done. The only way this argument could really be challenged is if you're not allowed to use the inverse of the Isosceles Theorem. But if that's in your book and has been proved earlier (or if your teacher does not require you to prove it), then everything should be fine.