Since you are given such a problem, presumably you know that "extrema" for a function, f(x), occur at "critical points"- where the derivative, f'(x), is 0 or does not exist. Such a critical point will be a "local maximum" if the second derivative, f''(x), is negative and a "local minimum" if it is positive. If the second derivative is 0, that test does not tell you anything.
So now the ball is back in your court- what are the first and second derivatives of \(\displaystyle \frac{16}{x^2+ 1}= 16(x^2+ 1)^{-1}\)?