No, a true proportion states that two ratios are equal.
8/10 = 4/5
2/5 = 4/10
5x-8y=1...(1)
3x+6y=-21...(2)
(1)*3:
15x-24y=3...(3)
(2)*5:
15x+30y=-105...(2)
(3)-(2):
15x-24y-(15x+30y)=3-(-105)
15x-24y-15x-30y=108
-54y=108
y=-2
The decimal gives after the 9. There are four places behind the decimal between the two numbers being multiplied so there have to be 4 places behind the decimal in your answer.
-5^4
= -625
-5 • -5 • -5 • -5 = -625
