$\begin{array}{l}a)\,\,3a^2 - 6ab + 3b^2\\=3(a^2 - 2ab +b^2)\\=3(a - b)^2\\b)\,\,x^2 - y^2 - 5(x - y)\\=(x-y)(x+y) - 5(x - y)\\= (x -y)(x +y-5)\\c)\,\,4 - x^2 + 2xy - y^2\\=2^2 - (x^2 - 2xy + y^2)\\=2^2 - (x - y)^2\\=(2 - x + y )(2 + x - y)\\d)\,\,x^2 + x - 6\\=x^2 - 2x + 3x - 6\\=x(x -2)+3(x -2)\\=(x-2)(x+3)\\e)\,\,18a^3b^2 - 9a^2b^3\\=9a^2b^2.2a - 9a^2b^2.b\\=9a^2b^2(2a - b)\\f)\,\,x^2 - y^2- 7x - 7y\\=(x - y)(x + y) - 7(x + y)\\= (x + y)(x - y - 7)\\g)\,\,16 - x^2 - 2xy - y^2\\=4^2 - (x^2 + 2xy + y^2)\\=4^2 - (x + y)^2\\=(4- x - y)(4+x+y)\\h)\,\,x^2 - 5x + 6\\=x^2 - 3x - 2x + 6\\= x(x -3) - 2(x -3)\\= (x -3)(x-2)\end{array}$
