Die Klammersetzung wird gar nicht passen. Du meinst sicher so:
(u-v)/(3u-6v)-(u+v)/(2u+4v)-uv/(u2-4v2)
= (u-v)/(3(u-2v) - (u+v)/(2(u+2v) - uv/((u-2v)(u+2v)) |HN:6*(u-2v)(u+2v)
= (2*(u-v)(u+2v) - 3*(u+v)(u-2v) - 6uv) / (6(u-2v)(u+2v))
= (u+2v)(v-u) /(6(u-2v)(u+2v))
= (v-u)/(6(u-2v))
Grüße