Triangle OAB is an isosceles triangle with vertex O at the origin and vertices A and B on the parabola y = 9-x^2

Express the area of the triangle as a function of the x-coordinate of A.

I'm having trouble finding the sides of all possible isosceles triangles that has its vertex in y= 9 -x^2. I know that the distance formula helps, so I can probably substitute the lengths by using A = 1/2 bh of a circle or maybe heron's formula. I will be glad if someone could help to figure out the equation for this.