First, we want to distribute the 2z to what's in the parentheses:
Then we need to distribute the negative sign outside of the parentheses in order to make it easier to solve and combine like terms:
Now we combine like terms. The '2z' and '-2z' cancel each other out, so:
Then we solve for z by dividing by -6 on both sides:
So now we know that z = -8.