An equilateral triangle, by definition, has all three sides of equal lengths.
For two points with the coordinates `(x_1,y_1)` and `(x_2,y_2)` the distance between them is
The length of the given side is `sqrt((2a)^2+(-2a)^2)=2|a|sqrt(2).`
Denote the third point coordinates as `x` and `y,` then the lengths of the other side are
`d_1^2=sqrt((x-a)^2+(y+a)^2)` and `d_2^2=sqrt((x+a)^2+(y-a)^2).`
And `d_1=d_2=2|a|sqrt(2).` This gives us the system of two equations with two variables.
Both sides of both equations are nonnegative, so we can square both equations and will obtain
Subtract the first from the second and obtain
`0=4ax-4ay,` or `x=y` (if `a!=0`).
Now substitute this into the first equation and obtain
`8a^2=2x^2+2a^2,` or `x^2=3a^2,` or `x=+-|a|sqrt(3).`
So for `a!=0` there are two solutions, `(|a|sqrt(3),|a|sqrt(3))` and `(-|a|sqrt(3),-|a|sqrt(3)).`
For `a=0` the only solution is `(0,0).`