Question

In the diagram, polygon ABCD is flipped over a line of reflection to make a polygon with vertices at A′, B′, C′, and D′. Points A', B', and D' are shown, but the line of reflection and point C′ are not. What are the coordinates of C'′ ?

Answer 1

when flipped over a line of reflection the lengths are still the same
the point to the line of reflection is the same length as the line of reflection to the reflected position
the distance from the original point to the reflected point is twice the distance from the original point to the line of reflection

cannot see your polygon.
here is an example