Length of AB on Parabola 4x2+7x-1 - Find the Solution

[chillfactor]

Homework Statement

points A and B are on the parabola f(x)=4xsquared + 7x-1, and the origin is the midpoint of AB, find the length of AB

Homework Equations

The Attempt at a Solution

I have spent time drawing out graphs and such but I am totally lost as to where to begin

 

[Mentallic]

Start off by letting the coordinates be $$A(x_A,f(x_A))$$ and $$B(x_B,f(x_B))$$. Now, what would be the midpoint of of AB in terms of these coordinates?