Assume this reads f(x)=6/(x^2+3).
The expression x^2+3 is minimum when x=0, so f(x) is maximum at x=0, f(0)=2.
Whether positive or negative, the square of x is positive so f(x)<2 for every value of x other than x=0. Therefore f(x) is concave downward (maximum) in the vicinity of x=0 and f(0) is the summit of a hill. The rest of the curve gets closer to the x axis as x increases positively or negatively.