Two dippers touch the water of a ripple tank in step…

At P. $\Delta x = 12.0\ \text{cm} - 9.0\ \text{cm} = 3.0\ \text{cm}$. In wavelengths, $\dfrac{3.0\ \text{cm}}{1.2\ \text{cm}} = 2.5$. Two and a half wavelengths is a phase difference of $5\pi$, so the ripples are out of phase. They cancel: $A = 0\ \text{mm}$. The water at P hardly moves.

At Q. $\Delta x = 12.4\ \text{cm} - 10.0\ \text{cm} = 2.4\ \text{cm} = 2\lambda$. The ripples are in phase: $A = 2 \times 2\ \text{mm} = 4\ \text{mm}$.