I thought that I had asked about another possible error in this same GRE study guide (months later, I am still in the same book!). The other error I remember was something like a 3 in a formula that was supposed to be ^3.

Hilarity did not ensue.

This error was in Chapter five, #17, page 124, explained on 127, of Manhattan Prep GRE Algebra. The image I shared was of my understanding of the problem. The actual problem, taken directly from the book is:

It seems they wrote the problem wrong. On page 115 they explain:

They explain you cannot be expected to multiply this on the GRE, even with a calculator, therefore you must look for a pattern in the powers of three.

I usually skip the lessons and go straight to the problems. Maybe if I read that part I would catch their errors faster. If I have needed to stop and figure out three mistake in the first book, how worth my time is this highest-rated study guide?