We need not consider a whole triangle but just point B.
Before reflection we know that [tex]B(5,9)[/tex].
Reflecting B over [tex]y=1[/tex] is relatively easy. First because its a reflection over the horizontal line the only coordinates that will change are y coordinates, while x coordinate will not change so half of the reflection is already done for us,
[tex]B(5,a)[/tex]
Now to what has changed, well currently the distance between 9 and 1 on the y axis is 8 up. But because we are reflecting the a must now be 8 down from 1 which means [tex]1 - 8 = -7[/tex] so our point is now [tex]\boxed{B(5,-7)}[/tex].
Hope this helps :)
