Question 56230
3(2 + x)= 5x - 4.2


6 + 3x = 5x - 4.2
  - 3x = -3x 

  6    = 2x  - 4.2
  +4.2 =     + 4.2

10.2   = 2x

 5.2   = x 

You did everything right except for the last step.  You didn't divide correctly.  The answer should be 5.1.
FYI, a good check of your work is to go  back to the original equation and put the solution in for the variable.  If it works out, you know the answer is right.