First, you need to calculate the resultant force:
R = m ¡ a
  = (12.3 + 5.1) ¡ 1.5
  = 26.1 N
Then, you can calculate the force of friction:
R = F - Fâ
Fâ = F - R
  = 33 - 26.1
  = 6.9 N
Now, we know that:
Fâ = Ο¡m¡g
Therefore we can solve for Ο:
Îź =Â Fâ/mg
  = 6.9 / (17.4 ¡ 9.8)
  = 0.40Â
The coefficient of dynamic friction is Ο = 0.40