Your question is:
[tex]\frac{8u}{u^2-18u+81} -\frac{72}{u^2-18u+81}[/tex]
Then, the above formula equate:
[tex]\frac{8u-72}{u^2-18u+81}[/tex]
Factor out 8 from the numerator we get:
[tex]\frac{u-9}{u^2-18u+81}[/tex]
Factorization of the denominator:
[tex]\frac{u-9}{(u-9)^2}[/tex]
Simplifying the numerator and the denominator by (u-9) we get:
[tex]\frac{1}{(u-9)}[/tex]
