\[A( - 1;\ - 1),\ \ \ B(2;\ 4):\]
\[AC = BC;\ \]
\[AC^{2} = BC^{2};\]
\[( - 1 - x)^{2} + ( - 1)^{2} = (2 - x)^{2} + 4^{2}\]
\[x^{2} + 2x + 1 + 1 = x^{2} - 4x + 4 + 16\]
\[6x = 18\ \ \ \]
\[x = 3.\]
\[Ответ:\ \ (3;\ 0).\]