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 = mx + 6\). Now we need to find the gradient.

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.

Stephen R. Wassell, Superexponentiation and Fixed Points of Exponential and Logarithmic Functions, Mathematics Magazine, Vol. 73, No. 2 (Apr., 2000), pp. 111-119 ...