We will use Boyle's law. P = 1/V which simply means pressure is inversely proportion to volume at a constant temperature. We will assume temperature here does not change. Since we have two pressure/volume points, we use this version of Boyle's law.
P1 V1 = P2 V2
P is pressure and V is volume
In our case:
P1 is 736, V1 is 4.2 while V2 is 9.0 but P2 is unknown. This is what we need to find out.
736 * 4.2 = P2 * 9.0
P2 = 736 * 4.2 / 9.0
P2 = 3091.2 / 9.0
P2 = 343.5
Therefore the new pressure 343 mm/hg
Here we see that the increase in volume causes decrease in pressure in total agreement with Boyle's law.