I was wondering if the above plagal cadence in Eb major was correct or not. Also, are the stem directions right? And am I right to assume that I had to write it in only 2 part? or is it actually 4 part?
The plagal cadence,
IV–I, would in fact have
A♭–E♭in the bass. Although your soprano
Gdo fit the chords, it's a pretty big leap (a perfect fifth) for the soprano to make.
C–B♭may be better, or perhaps
A♭–G(but use the lower octave if you go that route).
Your stem directions are correct.
As for whether this should be a two- or four-part exercise, that depends on the assignment; you'll have to consult your instructor (or the worksheet) for that one. If it is two-part, your instructor might prefer you to use the
Gas you have written to fill out the chords with the root and third. (Just doublecheck what is being asked of you!)
A plagal cadence in Eb will be an Ab chord followed by an Eb one. Your bass line is correct. The melody notes fit the chords. Beyond that, I'm afraid I can't give you many marks.
If there are going to be two more voices added (this sort of exercise is usually asking for 4-part writing) the stem directions are correct. But as it stands, with just two voices, the notes in the upper stave should have stems down. And are those melody notes, with the jump up, REALLY what you'd sing to the 'A-men' at the end of a hymn?
I think the requirement is for something like one of these two possibilities. (And I'm not doing any MORE of your homework for you!)
Contrary motion helps with a full sound, and can help avoud consecutives. But don't get obsessed with it. Particularly in a plagal cadence with it's feeling of 'settling down' into the tonic chord rather than having a rising leading note.