2. (6 points) $\square \square \square \square \square+\bigcirc \bigcirc O=39$ liters
■■■+○○○=27 liters
■ represents $\qquad$ liters, ○ represents $\qquad$ liters.
【Solution】Solving: Subtracting the two equations yields:
$$
\begin{array}{l}
(39-27) \div(5-3) \\
= 12 \div 2 \\
= 6 \text { (liters) } \\
27 \div 3-6 \\
= 9-6 \\
= 3 \text { (liters) }
\end{array}
$$

Answer: ■ represents 6 liters, ○ represents 3 liters. Therefore, the answers are: $6 ; 3$.