I'm not sure even this is correct. Live action and animation had been combined in silent movies, e.g. Gertie the Dinosaur.
Gertie is a short film, isn't it? The question was something like "What is the first feature-length film to feature live action and animation?"
Having said that, I Googled Gertie the Dinosaur
after reading your post and discovered that Anchors Aweigh
could be the wrong answer, anyway. The actual first feature film may be The Three Caballeros