K6

a)
2x^2+6x=2x\left(x+3\right)
b)
x^2-4=\left(x+2\right)\left(x-2\right)
c)
x^2-2x+1=\left(x-1\right)^2
d)
9x^2-25=\left(3x-5\right)\left(3x-5\right)