ymolchi is 'to wash yourself' and so 'myself' is the missing extra word in English, it is one of the accepted alternative answers here. The word to wash something is 'golchi' which is in this unit but we've had to disable at the moment because of a Duolingo system wide problem with the apostrophe.
What about simply to say, "Yes, I washed"? Maybe a regional incorrect way of using English, but much more common than "I have washed." And, this would still work with the missing word "myself."
There is a difference between 'I have washed' and 'I washed'.
'I have washed' refers to one action in the immediate past.
'I washed' refers to an action at some defined or undefined time in the past.
eg. 'What have you done this morning? I've washed and dressed.'
'What did you do after getting up Thursday morning? I washed and dressed.'
For first example we use 'wedi' in Welsh eg 'Dw i wedi ymolchi a gwisgo'
For the second example we use either 'Ymolchais i a gwisgais i' in South Wales or 'Mi wnes i ymolchi a gwisgo' in North Wales.
to Cynphony: There is a difference between British and American usage here. "Wash up" in the US means, or can mean, to wash your hands, face etc. "Wash up" in British English means to do the washing-up i.e. clean dirty dishes. So "ymolchi" could be translated as "wash up" in America but definitely not in Britain, since "ymolchi" means to wash oneself, not something else.