Guessing it is a laptop? If it is a laptop, they should have a utility in your systray that will allow you to set how 3D applications are managed. You may need to set a specific profile for the game to make sure it runs it on the 3D chip and not the Intel. The 8870m isn't the greatest (and the 8870m/R9 270x is a bit lower on the scale), but it should be enough to run the game around 30 FPS with the lower graphics options set (maybe start with the laptop defaults, and adjust settings from there as needed).