The stationary points of a function are the points where its derivative is zero. In this case

3x^2-48 x + 192 = 0 OR x^2 - 16 x + 64

Find x and corresponding y from the given function.

To know if the stationary point is concave upward or down ward find the second derivative. If it is negative then the at that point the function will be concave downward and if it is positive then it will be concave upward.