Exercise 55 Page 674

The transformation $R_{y=1}\circ T_{<2,\text{-}1>}\left(△ABC\right)$ can be written as $R_{y=1}\left(T_{<2,\text{-}1>}\left(△ABC\right)\right).$ This means that $△ABC$ will be translated $2$ units to the right and $1$ unit down. Then, the image will be reflected using $y=1$ as the line of reflection.

Let's start by finding the coordinates of the vertices of $△ABC.$