The graph of the quadratic function y = a x 2 + b x + c has a minimum turning point when a> 0 and a maximum turning point when a a <0. The turning point lies on the line of symmetry.

From the graph, we can also see that the y-intercept is 6, therefore we can say that the equation of the straight line is y = m x + 6. Now we need to find the gradient.

Ivan Baggs, Functions with a Closed Graph, Proceedings of the American Mathematical Society, Vol. 43, No. 2 (Apr., 1974), pp. 439-442 ...