Suggestion: This assignment is difficult to perform/complete.
I understand that you want to render the structure of the Indonesian sentence in the translation, but "This assignment is difficult to to be performed" just doesn't work in English. Or does it?
Nobody that I have heard uses a structure like the answer. It may be possible in English, but it is not something anyone would use.
Come on Duo, there has got to be a less clunky English translation you'll accept.
Perhaps the word "task" is more reflective of the word "tugas"?
It should be 'this task is difficult to perform'
"This sentence is difficult to be translated"