Given
The triangle DEF.
To graph the triangle DEF under a reflection across the line y=-1.
Now,
It is given that, the triangle DEF.
The graph of the triangle DEF under the reflection across the line y=-1 is,
Here, the vertices of DEF is (1,1), (3,3), (6,1).
Then, for y=-1, the vertices of D"E"F" is,

Hence, the coordiantes of D"E"F" are (1,-3), (3,-5), (6,-3).