Answer:
v₁ = 0.29 [m/s]
Step-by-step explanation:
This problem can be solved by preserving the momentum. This can be represented by means of the following equation.

where:
m₁ = mass of the astronaut = 64 [kg]
v₁ = velocity of the astronaut [m/s]
m₂ = mass of the rock = 5.4 [kg]
v₂ = velocity of the rock = 3.5 [m/s]
Now replacing:
![64*v_(1)=5.4*3.5\\v_(1)=0.29[m/s]](https://img.qammunity.org/2022/formulas/physics/college/n2wxa178rrudxzhm0cs93l75rczxf3t9g1.png)