I haven't checked to see if either are available, but there is/was an
answer key for Machen that was done by Ziodhiates (pub. by his outfit: AMG,
1976). I also have an early draft of a workbook for Machen that was done by
David Thompson at Marion College (also 1976). I'd be surprised if either
were still available, however.
A far better choice is to use Bill Mounce's textbook (Basics of Biblical
Greek, Zondervan, 1993) and accompanying workbook. It is far better
pedagogically as well as much cheaper than Machen.
Rodney J. Decker, Asst. Prof./NT Baptist Bible Seminary
email@example.com Clarks Summit, PA