\(-9x^2+y^2=y^2-\left(3x\right)^2=\left(y-3x\right)\left(y+3x\right)\)
\(\left(x-3\right)^2-16y^2=\left(x-3\right)^2-\left(4y\right)^2=\left(x-3-4y\right)\left(x-3+4y\right)\)
\(\left(x+5\right)^2-\left(y-7\right)^2=\left(x+5-y+7\right)\left(x+5+y-7\right)=\left(x-y+12\right)\left(x+y-2\right)\)
\(16\left(2x+3\right)^2-9\left(5x-2\right)^2=\left(8x+12\right)^2-\left(15x-6\right)^2=\left(8x+12-15x+6\right)\left(8x+12+15x-6\right)=\left(18-7x\right)\left(23x+6\right)\)
17) -9x² + y²
= -(9x² - y²)
= -(3x - y)(3x + y)
18) (x - 3)² - 16y²
= (x - 3 - 4y)(x - 3 + 4y)
19) (x + 5)² - (y - 7)²
= [(x + 5) - (y - 7)][(x + 5) + (y - 7)]
= (x - y + 12)(x + y - 2)
20) 16(2x + 3)² - 9(5x - 2)²
= [4².(2x + 3)²] - [3²(5x - 2)²]
= (8x + 12)² - (15x - 6)²
= [(8x + 12) - (15x - 6)][(8x + 12) + (15x - 6)]
= (18 - 7x)(23x + 6)
