` a) ` ` 9(a + b)^2 - (a + b) `
` = (a + b)[9(a + b) - 1] `
` = (a + b)(9a + 9b - 1) `
` b) ` ` (mx + my) + (3x + 3y) `
` = m(x + y) + 3(x + y) `
` = (x + y)(m + 3) `
` c) ` ` (12xy - 6x) - (2y - 1) `
` = 6x(2y - 1) - (2y - 1) `
` = (2y - 1)(6x - 1) `
` d) ` ` (7xy^2 - 5x^2 y) + (5x - 7y) `
` = -xy(5x - 7y) + (5x - 7y) `
` = (5x - 7y)(1 - xy) `
` e) ` ` 2x(x - y) - (4x - 4y) `
` = 2x(x - y) - 4(x - y) `
` = (x - y)(2x - 4) `
` = 2(x - y)(x - 2) `
` f) ` ` (15xy + 20x^2 y) + (3 + 4x) `
` = 5xy(3 + 4x) + (3 + 4x) `
` = (3 + 4x)(5xy + 1) `
` g) ` ` 2019(x - y)^2 + 673(y - x) `
` = 2019(x - y)^2 - 673(x - y) `
` = (x - y)[2019(x - y) - 673] `
` = (x - y)(2019x - 2019y - 673) `