Sorry, you've jumped from local max/min to absolute max/min, before we continue, could you please clarify whether you're being asked to find any turning points in the function or are you being asked to show that the range of the function is all Reals?
edit: I have a strong hunch that it's the first. Ok so if you were given any function, such as a quadratic, what would be the process to show where the turning point occurs?