1. 7$x^{2}$ - 7xy -4x+4y
⇔ 7x (x-y) -4 (x-y)
⇔(x-y) (7x-4)
2. 25 - $m^{2}$ - 2mn - $n^{2}$
⇔25- ($m^{2}$ + 2mn + $n^{2}$ )
⇔25- $(m+n)^{2}$
⇔(5-m-n) (5+m+n)
4. $p^{2}$ - 2pq -9 + $q^{2}$
⇔$p^{2}$ - 2pq+ $q^{2}$ -9
⇔$(p-q)^{2}$ -9
⇔( p-q-3) (p-q+3)
5. $-16^{2}$ + 8xy - $y^{2}$ + 49
⇔49-($16^{2}$ - 8xy + $y^{2}$ )
⇔49-$(4x-y)^{2}$
⇔(7-4x+y) (7+4x-y)
6. $36a^{2}$ - $(a^{2}$ + 9)$^{2}$
⇔(6a-$a^{2}$ -9) (6a+ $a^{2}$ + 9)
⇔-($a^{2}$ - 6a +9 )(6a+ $a^{2}$ + 9)
⇔- ($(a-3)^{2}$ ) (a+3)$^{2}$
7. 10ax - 5ay +y - 2x
⇔5a(2x-y)-(2x-y)
⇔(2x-y)(5a-1)