50÷[(4·÷2)]=y
what is the value of y?

Y=25

Isn't it 25? Order of Operations says P.E.D.M.A.S... Parenthesis, Exponents, Division, Multiplication, Addition, and Subtraction. 

Remove 50 and y from the equation for now.
[(4·5)-(36÷2)]

Solve what is in the parenthesis first: 4*5 = (20)   36/2 = (18)
Now you should have [(20)-(18)].
[(20)-(18)] = 2

Now you can add 50 back to the back to the beginning of the equation.
50÷[(20)-(18)]
50÷2
25

If this is incorrect I sincerely apologize!

$12000*0.08 = 960
960*6= 5760
He has $5760 after 6 years.