Translation:I practice the piano and the guitar.
Understand you but the makers of the app are our teachers and whilst I have no bother with learning the japanese in their word order etc., I do take issue with the poor standard of english. Although we're all given to some degree of bad grammar there needs to be a high standard of english or we will misunderstand the meaning of the language we are learning and end up with a lot of embarrassing (or worse) momentsvwhen attempting to communicate with native japanese.
Under "Report," there needs to be a "The English sentence is unnatural or has an error." because "I practice the piano and the guitar." sounds awkward. I would more naturally say that "I practice both the piano and guitar." but that natural translation is marked WRONG. The particle "the" is naturally carried from the first object to the second by the "and" conjunction. I have already submitted that this translation be accepted and now habitually put in the awkward one that is "accepted." As it is assumed that the Japanese sentences may be equally poor, how many of these awkward sentences am I learning in Japanese without knowing that the structure is unnatural to a native speaker? I am trusting Duolingo to teach me correctly.