ok, i don't know what the stroke of a cylinder is (should i be embarrassed about that?), so i can't help you with the first, but for the second, this is the way to go.

For a cylinder, the volume, V, is given by:

where is the radius of the base of the cylinder, and is height of the cylinder.

when the height is 6, the volume is 300

.......divide both sides by 6

So, when the height goes to 2, the volume will be:

Now by Boyles' Law:

So the new volume is 100 cubic inches and the new pressure is 30 psi