Q: A rectangle is inscribed with its base on the x-axis and its upper corners on the parabola y= 3-x^2. What are the dimensions of such a rectangle with the greatest possible area?

My Solution:

Okay, I understand I have to find a rectangle with the greatest size that has to corners in f(x) = 3-x^2. But I really do not know how to even start, would someone please give me a hint?