The answer is -17
The negative 7 and positive 7 add up to 0. They are opposites which cancel out. Think of it like being 7 dollars in debt (negative 7) and that debt is fully canceled out by the positive amount of money (positive 7). Put another way, the negative 7 means you go down 7 while the positive 7 means you go up 7.
After the first two terms cancel to 0, the -17 is the only thing left. Adding 0 to any number keeps that number the same.
Adding the negative and the same number that is opposite, like positive, would cancel it out. So -7+7=0, since opposites cancel eachother out. The answer is what's left: -17, and ignore the addition sign since there is nothing on the other side to connect it.
Or you could do:
because if you look at the chart you can see a pattern and compare it to the question he simplify out your
but i think the formula is f(x)=6x