Maths problem: how to do 4a and b, thanks, I still don't understand.?

(4a)

The change of the volume of water after the sphere is put into the tank
= New volume - Initial volume
= π*(7)^2*(7-4)
= 147π cm^3

The volume of the part of the sphere that is not immersed in the water
= The volume of the sphere - The change of the volume of water
= 4/3π*(5)^3 - 147π
= 166.667π - 147π
= 19.67π cm^3

(4b)

Let x be the radius of the circle at the water level.
According to the Pythagorean theorem,
(5cm - 3cm)^2 + x^2 = r^2
4 + x^2 = 5^2
x = sqrt(21)

The area of the water surface
= π(7)^2 - x
= π(7)^2 - π(sqrt(21))^2
= 49π - 21π
= 28π