Answer:
42x^2
Step-by-step explanation:

Since you have y alone on one side of the equation......
Use substitution and plug in 1/2x-4 for y in the other equation.

Distribute

Combine like terms

Add 8 to both sides

Divide by 4
and 
Plug this value in for x

Simplify

Multiply everything by 8 and simplify

Divide.....

Your answers are:

They are exactly the same thing
Answer:
$38.06
Explanation:
Multiply $31.20 by 0.22 (22%)
Which equals 6.864
Add that to $31.20 and you get 38.064
Then you round it and get $38.06