It looks to be at #x=3#
Notice that #x=3# is the x coordinate that corresponds to where the vertex of the parabola is, #x=3# is also the axis of symmetry. This is the turning point of the function. Notice to the left of this point that the gradients of tangents drawn there are negative and to the right of this point they are positive. At the vertex the gradient is neither negative nor positive. This has a gradient of #0# and is often called a stationary point. This is the point where the gradient changes. For a simple quadratic like this, the point could be found using the vertex form of the quadratic.
#y=(x-h)^2+k#
Where #h# is the axis of symmetry.
For other functions we would use the first derivative equated to zero. This would identify all points that have a zero gradient.
