Net Force on 2m Mass: Prove/Disprove?

[Cali37]

Homework Statement

Three masses m, 2m and 3m are placed on a horizontal surface. A force F is shown acting on m3. Assuming μs,k is the same between all of the masses and the horizontal surface, answer the following:
a) Find the expression for the net force on the 2m mass.
b) For F constant, μs,k =0 then the answer to (a) must decrease. Prove or Disprove.

Homework Equations

F=ma

The Attempt at a Solution

F=ma
μs,k =m+2m+3m

 

[haruspex]

There appears to be a vital diagram missing.

μ_s,k =m+2m+3m

That makes no sense. The quantity on the left is a dimensionless coefficient, while that on the right is a mass.