That's a yellow bellied slider, Straight lined on the scutes are red ears and yellow bellies, while cooters have curved, c shaped markings. Also the large yellow blotch behind the eye is a true sign.
Yup, you guys got it! That yellow blotch, as Mark references, is the dead give away. To help others, since there are other species with yellow markings, notice how this, and all Yellow Bellied Sliders, Trachemys scripta scripta, have that big yellow blotch that is in the shape of an "S". That is what sets it apart from other Trachemys and Pseudemys.