Answer: -18
Big Idea:
When given an expression, plug in the value of the variable that should be given to you. Then, evaluate/simplify by applying the technique of PEMDAS.
Step-by-step solution:
In this case, we are given the expression
; the value of x is -6.
Put in -6 for x:

Then, multiply.

Therefore, the value of 3x when x = -6 is -18.