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:

Answer: y = 1
Step-by-step explanation:
9x + (3 x 1) = 12
(50-25)/2
25/2
12.5
25+12.5=37.5
Answer: To find the finance charge you multiply the balance times 0.23 / 12 .
To find the interest charge to an account, you have to multiply the balance by the percent rate (APR).
The problem in this case is that interest is charged monthly, but the rate is yearly.
To fix this problem, just divide the rate by 12.
0.23 / 12
The answer to the question