imc

Consider a point $(x,y)$. Reflecting it about the $x$-axis will map it to $(x,-y)$, and rotating it counterclockwise about the origin by $90^{\circ }$ will map it to $(y,x)$. The operation that undoes this is a reflection about the $y=x$, so the answer is $\boxed{\text{reflection about the line }y=x}$.