Let $R$ be a vertex of the rhombus on the side $BC$.
Then, since $PR$ is parallel to $AC$, we see that $\triangle{BPR}$ and $\triangle{BAC}$ are similar.
So, we have $$BP:BA=PR:AC\implies 12-s:12=s:6\implies s=4$$
Let $R$ be a vertex of the rhombus on the side $BC$.
Then, since $PR$ is parallel to $AC$, we see that $\triangle{BPR}$ and $\triangle{BAC}$ are similar.
So, we have $$BP:BA=PR:AC\implies 12-s:12=s:6\implies s=4$$