a, \(x^2-3xy+x-3y\)
= \(\left(x^2+x\right)-\left(3xy+3y\right)\)
= \(x\left(x+1\right)-3y\left(x+1\right)\)
= \(\left(x+1\right)\left(x-3y\right)\)
b, \(7x^2-7xy-4x+4y\)
= \(\left(7x^2-7xy\right)-\left(4x-4y\right)\)
= \(7x\left(x-y\right)-4\left(x-y\right)\)
= \(\left(x-y\right)\left(7x-4\right)\)
c, \(x^2+6x-y+9\)
= \(\left(x^2+6x+9\right)-y\)
= \(x^2+2.3x+3^2-y\)
= \(\left(x+3\right)^2-y\)
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