Both F(x) and G(x) are quadratic equations. The graphs of the two functions are known as parabolas. All four choices for the equation of F(x) are written in their vertex form. That is:
The point is the vertex of the parabola, and
The value of determines the width and the direction of the opening of the parabola. means that the parabola opens upward. means that the parabola opens downwards. The opening becomes narrower if the value of increases.
Use tan inverse 5/12, that should be the answer because if the triangles are similar, they have the same angle measures. You can also use sin inverse 5/13 or cos inverse 12/13. I recommend using two of the three, one to get the answer and other to check.