If I have the assets, liabilities and capital stock figures, how can I use these figures to find the retained earning figure?

The following link should help you out

Online Accounting Course | Accounting Equation

Assets = Liabilities + Owner's Equity

Owner's Equity = Assets – Liabilities

Also, Owner's Equity = Capital Stock + Retained Earnings

So,
Capital Stock + Retained Earnings = Assets – Liabilities
Retained Earnings = Assets – Liabilities – Capital Stock