When you substitute values into an algebraic equation, you need to input the values exactly how they are given. "w" must go where there are "w"s, and "v" must go where the "v"s are, otherwise you will get an incorrect answer. All values must also carry over their signs--they are one unit--and must be inserted as such, which is why - (-8) is correct, but not - (8) or - 8. (This is because those two negatives will cancel each other out and become +8 when solving). The parenthesis around the values are important because they protect the original values of the variables, which is why (-2)2 is correct. In the case of (-2)2, it also signifies that they are "attached" by multiplication, and when solving would become -4. Without signifying that the variables are separate from the rest of the equation via the parenthesis, it becomes very easy to solve it incorrectly.
