They got back in the Lunar Explorer Module
Answer:
I think it is 2.5 kg.
Explanation:
I can say you do:
Fnet = m * a
So, you want to find mass:
m = Fnet / a
So, the answer you will receive is 2.5
Answer: Total work done on the block is 3670.5 Joules.
Step by step:
Work done:

With F the force, d the displacement, and theta the angle of action (which is 0 since the block is pushed along the direction of displacement, and cos 0 = 1)

Given:
F = 75 N
m = 31.8 kg
Final velocity 
In order to calculate the Work we need to determine the displacement, or distance the block travels. We can use the information about F and m to first figure out the acceleration:

Now we can determine the displacement from the following formula:

Here, the initial displacement is 0 and initial velocity is also 0 (at rest):

Now we still have "t" as unknown. But we are given one more bit of information from which this can be determined:

(using vf as final velocity, and tf as final time)
So it takes about 6.44 seconds for the block to move. This allows us to finally calculate the displacement:

and the corresponding work:

Answer: 9.375m/s
Explanation: This is an inelastic collision, so the formula is mv1+mv2=(m1+m2)v-final. My work is a follows:
2500kg(12m/s) + 700kg(0m/s)=(2500kg+700kg)(v-final)
30,000kg(m/s)+0=3200kg(v-final)
30,000kg(m/s)/3200kg=9.375m/s