You can solve 9/x-4/y=8 for x or for y but NOT at the same time.
Solving for x: 9/x-4/y=8 Mult all 3 terms by xy to eliminate the fractions.
9(xy)/x - 4xy/y = 8xy => 9y - 4x = 8xy, or 9y = 4x + 8xy = 4x(1+2y)
then 9y = x [ 4(1+2y) ]
9y
therefore x = ------------
4(1+2y)
Solve for y using a similar approach.