-w is the variable, so our goal is to get it on the left side of the equal sign and keep the 15 on the right side.
distribute the -2 to each term inside the parentheses. the equation should now look like this:
2w - 6 = 15
now, we must transpose the -6 to the other side of the equation. remember, we're moving it across the equal sign, so it becomes a positive 6.
2w = 21
now divide each side of the equation by 2 so the variable is alone.
w = 10.5
hope i helped!