# How do you find #a# for the derivative of #f(x)=x^2-ax# if the tangent has a y-intercept of -9 at the vertex?

##### 1 Answer

Jul 12, 2018

#### Explanation:

Given the equation of the parabola:

we can immediately determine for symmetry reasons that the vertex has coordinates:

Since the tangent to the parabola at the vertex is horizontal (because it is a local minimum) it follows that:

That is:

graph{x^2-6x [-7.79, 12.21, -11.24, -1.24]}