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Step 1: FBD
Draw individual free-body diagrams for the two blocks. For this, define a coordinate y that describes the motion of the lower block. Let's say you choose y to be positive to the left. Be sure that the directions of the spring force on the top block and the dashpot force on the lower block are consistent with your definitions of x and y.
Step 2: Kinetics - Newton/Euler
Write down the Newton equations for the two blocks. Be sure to abide by the sign conventions defined for x and x when writing down these equations.
Step 3: Kinematics
The kinematics that you need here are to relate x and y. For x defined positive to the right and y defined positive to the left, we have x = +y .
Step 4: Equation of motion
Combine your equations from Steps 2 and 3 to end up with the differential equation of motion for the system in terms of the coordinate x.