Calculate the number of models of a KB
Here's how to count the models:


There are two options for $a$. Denote the option not chosen by $\bar{a}$ (so if $a = x$ then $\bar{a} = y$, and if $a = y$ then $\bar{a} = x$).
There is one option for $p(a),q(a)$: both have to hold.
There are three options for $p(\bar{a}),q(\bar{a})$.
There are two options each for $b,c$.


In total, we get $2 \cdot 1 \cdot 3 \cdot 2 \cdot 2 = 24$.