The third vertex might be (a,0).
Consider A = (0,0) B = (2a,0) and C = (a,0)
For an isosceles triangle, two sides and two angles will be equal. Based on that theory, side BC = CA.
So calculating the distance BC = (2a-a)^2 +(0-0)^2 = a^2
Calculating the distance CA = (a-0)^2 + (0-0)^2 = a^2
The sides are equal that is a^2