Given that you mention the divergence theorem, and that part (b) is asking you to find the downward flux through the disk , I think it's same to assume that the hemisphere referred to in part (a) is the upper half of the sphere .
a. Let denote the hemispherical <u>c</u>ap , parameterized by
with and . Take the normal vector to to be
Then the upward flux of through is
b. Let be the disk that closes off the hemisphere , parameterized by
with and . Take the normal to to be
Then the downward flux of through is
c. The net flux is then .
d. By the divergence theorem, the flux of across the closed hemisphere with boundary is equal to the integral of over its interior:
We have
so the volume integral is
which is 2 times the volume of the hemisphere , so that the net flux is . Just to confirm, we could compute the integral in spherical coordinates: