3(x+4) <or= 3x

3(x+4) \leq 3x

expanding brackets:

3x+12 \leq 3x

This is obviously never true.

You are trying to find a range for which the statement is true.

Simplify as if it were a normal equality. If at some point you multiply or divide by a negative number (in this case I don't think that is likely), remember to reverse the inequality.