The second is not too hard. The key is that f(x) * f(y) = f(y) * f(x), so f(x + y) + f(x - y) = f(x + y) + f(y - x). Therefore, f(x - y) = f(y - x). Then substitute x = 0.

To find f(7), start with small numbers. E.g., f(1) * f(0) = f(1) + f(1), which gives f(0).