washing machine drainage question

this is probably a really stupid question...
my housemate and i just connected our new washing machine, and it's working fine, but when it drains the waste water comes stright back up into the sink and also into the bath, which is in a different room. no water is washing back into the machine itself as far as i can see (and i guess it would stop working if that happened).
we've connected the waste pipe of the machine to one of the short pipes that stick out upwards above the u-bend of the pipe under the kitchen sink.
currently the unused sticky-out pipe (sorry for lack of technical names!) is still stoppered - should this be open to allow drainage to happen properly, or would this result in water going everywhere?
or does this suggest that there's a blockage further down?
the drainage in general from our bath and sinks has now decreased considerably, so i'm really hoping this is an obvious fix!

You have a blockage or partial blockage from the trap to outside drain. Try a good quality sink plunger to start with.The other spigot(sticky out thing) should not be opened. This has nothing to do with drainage configuration but is a second drainage inlet for if you were to connect a dishwasher.

thats great, thanks. i thought as much (the water in the bath is full of stones etc) but wondered if maybe it was something obvious that i hadn't done, as a first port of call!
cheers though.