u/AliasFutility

When washing machine is being used there is water being pushed up into the sink when it drains, and the sink also drains slow.

There was leaking from pipes beneath the sink but reconnected them firmer and most of the leaking is resolved. One of the pipes look to be missing a seal so looks like that is the remaining issue (guessing).

Is this configuration normal or has my plumber done something wrong?

Thanks so much.

Edit: I should mention one side of the house is all draining slowly - sinks and toilets after flushing. Other side with kitchen sink and washing machine are fine.