I'm not sure what your variables are, but I will try task 3 with you.
y=$500(in billions)
x= the number of years that have passed since 2009
percentage of decrease: 1 - % =
so your formula will look like this: y(1 - %)^x
Answer:

Step-by-step explanation:
First of all, the number before the decimal would be the whole number of the fraction.
So, 
Then, you would put 0.83 over a fraction.
So because you cannot simplify it anymore, it would be 
6y = 24 -3x
divide both sides by 6.
y = (x)/2 +4
I am not sure but I think the answer is B.
Divide 25/9 and you'll get the decimal 2.7 repeating boom decimal.