(x-81)/(9x-81) - (7x + 9)/(9x - x2)
= (x-81)/(9x-81) + (7x + 9)/( x2 - 9x )
= (x-81)/(9(x-9)) + (7x + 9)/( x(x - 9) )
= ( x(x-81)+ 9(7x + 9))/( 9x(x - 9) )
= ( x^2 - 81x + 63x + 81)/( 9x(x - 9) )
= ( x^2 - 18x + 81)/( 9 x(x - 9) )
= (x-9)^2 /(9x(x-9))
= (x-9)/(9x) , wobei x≠9.