Its determinant is -1 so it must be a reflection with no scaling. You can work out the line of reflection by plotting the points (1,0) and the image of (1,0) under the transformation. Draw a line through the origin and (1,0) (i.e. the x-axis) and a line through the origin and its image; what is the angle between these two lines? The line of reflection must bisect this angle; work out the gradient (and hence equation) of this line. Et voilà.