The distance from the bottom of the objective lens to the surface of the slide is called the

It's called the working distance .

if you already have the answer to (a), then just work out how many periods there are in 1 minute, and that's the answer to (c).

can be answered using momentum.

momentum = p

p1= mass1 * velocity1

p1 = p2

therefore

p1 = 6kg * 4m/s

p1 = 24kgm/s

p2=m2 * v2

24 kgm/s = 7kg * v2

v2 = 24 kgm/s / 7kg

v2= 3.42 m/s