Is the printed solution on page 161 not sufficient?
Acording to the statement of changes in equity we had a brought forward figure for retained earnings as at 31 December 2008 of $2,000 per the question
Then we had a material error of $500 discovered in 2009 that required a prior year adjustment and that reduced the $2,000 brought forward figure down to $1,500
Then there was this year’s retained earnings of $1,200 to add on and that gives us the figure of $2,700 to carry forward as at 31 December 2009