(4x+y-9)(4x-y+9)=16x²-(y-9)²
16x²-(y²-18y+81)
16x²-y+18y-81
(a+b+5c)(a+b-5c)=(a+b)²-25c²
a²+2ab+b²-25c²
(4x+y-9)(4x-y+9)=16x²-(y-9)²
16x²-(y²-18y+81)
16x²-y+18y-81
(a+b+5c)(a+b-5c)=(a+b)²-25c²
a²+2ab+b²-25c²