What is the equation for the line of reflection that maps the trapezoid onto itself?

##y = -1##

It is an isosceles trapezoid, which is just as well, otherwise it could not reflect onto itself. The line of reflection is the same as the mirror line which is the same as the axis of symmetry.

It runs horizontally exactly through the middle of the shape.

All horizontal lines have the equation ##"y = a value"##

In this case the line is ## y = -1##