Sorry, I had begun typing this answer on my own and came in and saw it was answered! I just thought I'd type out my walk-through steps...
At any rate, be careful with making sure you account for negative signs!
In your question, the function was defined as:
So yes, when asked for g(-3) you do substitute the value "-3" in for "x."
But remember, the function includes a negative sign before the "x." So, when substituting "-3" in for "x," your function would read:
g(-3) = -(-3)+6
g(-3) = 3 + 6
g(-3) = 9