Find the slope of the tangent line to the ellipse $\displaystyle \frac{x^2}{9} + \frac{y^2}{16} = 1$ at the point $(x,y)$.
slope =

Are there any points where the slope is not defined? (Enter them as comma-separated ordered-pairs, e.g., (1,3), (-2,5). Enter none if there are no such points.)
slope is undefined at

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