Answer:
G'(1,7), H'(-8,1), I'(-4,-5)
Step-by-step explanation:
The transformation matrix for reflection across the y-axis is an identity matrix with the x-multiplier negated:
![\left[\begin{array}{cc}-1&0\\0&1\end{array}\right]](https://tex.z-dn.net/?f=%5Cleft%5B%5Cbegin%7Barray%7D%7Bcc%7D-1%260%5C%5C0%261%5Cend%7Barray%7D%5Cright%5D)
Multiplying the coordinates by this gives ...
![\left[\begin{array}{cc}-1&0\\0&1\end{array}\right]\cdot\left[\begin{array}{ccc}-1&8&4\\7&1&-5\end{array}\right]=\left[\begin{array}{ccc}1&-8&-4\\7&1&-5\end{array}\right]](https://tex.z-dn.net/?f=%5Cleft%5B%5Cbegin%7Barray%7D%7Bcc%7D-1%260%5C%5C0%261%5Cend%7Barray%7D%5Cright%5D%5Ccdot%5Cleft%5B%5Cbegin%7Barray%7D%7Bccc%7D-1%268%264%5C%5C7%261%26-5%5Cend%7Barray%7D%5Cright%5D%3D%5Cleft%5B%5Cbegin%7Barray%7D%7Bccc%7D1%26-8%26-4%5C%5C7%261%26-5%5Cend%7Barray%7D%5Cright%5D)
That is, the reflected points are G'(1,7), H'(-8,1), I'(-4,-5).