Formula problem

i created a formula today and i struggle with the "shifting" (dont know how you call this in english)

the formula


( 0.9 * 1000 + 0.1 * (0.3 * 7000 - 0.7*5000)

i want to get the results as near as possible to +- 0. there are to values that you can adjust the 0.3 / 0.7 (30% 70%) the rest is static.

my first try was i divided it by 0.1, but this changed the whole result, and iam confused :/

(0.9*1000) / 0.1 + (0.3*7000 - 0.7 *5000)

So my first question is :
why is (0.9*1000) / 0.1 + (0.3*7000 - 0.7 *5000) != ( 0.9 * 1000 + 0.1 * (0.3 * 7000 - 0.7*5000)

and the 2nd question.

how can i get a results that is near +-0 if you can only change two values (0.3 / 0.7).

Edit : ups is should mention , that the formula above was just an example, so in this case its obviously to make the 0.3 => 0.01 and the 0.7=> 0.99. but i could give you another example what could happen

( 0.3 * 1000 + 0.7 * (0.3 * 7000 - 0.7*5000)
as you can see the weightning is now stronger on the right side.