12. Define a new operation: $x \odot y=18+x-a \times y$, where $a$ is a constant. For example:
$$
1 \odot 2=18+1-a \times 2 \text {. }
$$

If 2๑3=8, then $3 \odot 5=$ $\qquad$ , $5 \odot 3=$ $\qquad$
Reference answer: 1,11
Key points: Calculation - Defining new operations