What is the mistake in this problem?

The mistake is that 54 was not subtracted from both sides. It should be. Also, I have no idea what those shenanigans with the -3b = 60 simplification is, but you have to divide both sides by -3. b = 16

Step-by-step explanation:

This is how this is supposed to be solved.

 54 - 3b = 6

-54            -54

-3b = -48

(-3b)/-3 = -48/-3

b = 16

a is the right answer