A square has vertices at \(A(0,0)\), \(B(-9,12)\), \(C(3,21)\), and \(D(12,9)\).
The line \(\ell\) passes through \(A\) and intersects \(CD\) at point \(T(r,s)\), splitting the square so that the area of square \(ABCD\) is three times the area of \(\triangle ATD\).
Determine the equation of line \(\ell\).