If x = -3, we need to plug in -3 for x (in parenthesis if it isn't already).
You current expression is:
3x -4
Plug -3 in for x
3(-3) -4.
When multiplying a negative by a positive, your product ends as a negative (unless you multiply by 0).
3(-3) = -9
Your expression now is:
-9 -(4).
When subtracting a negative from a negative, pretend you're just adding positives with positives, except your value sign is negative.
-9 -(4) = -13.
-13 is your solution.
I hope this helps!