If x/y=4, x-y=9, and y-x=-9, what is x & y?

Answer 1

x/y=4, means x=4y
substitute 4y for x in x-y=9
to get
(4y)-y = 9

Can you continue?

Note: the third equation (y-x)=-9 can be derived from the second (x-y)=9, so it gives no new information.

Answer 2

3y