The point (x,y) reflected in the line Ax+By+C=0 is the point
Since your line is
or
A=1, B=1, C=-1/2
that simplifies to
So in the ellipse equation, replace x by
and replace y by
and you'll get an equation of another ellipse.
I won't do that work for you, but here is the graph. The red
ellipse is the given one. The green one is the one reflected
in the line:
Edwin