Simplify the following:

(3x+y-5)(3x-y-5)

(3x+y-5)(3x-y-5) = (3x-5+y)(3x-5-y)

Let (3x-5)=a

Then, the given expression = (a+y)(a-y)

= a² - y² = (3x-5)² - y²

= [(3x)² - 2(3x)(5) + (5)²] - y²

= 9x² - 30x + 25 - y²