a(a+b)-b(a+b)

Answer 1

a ( a + b ) - b ( a + b ) = ( a + b ) ( a - b ) = a² - b²

OR

a ( a + b ) - b ( a + b ) = a ∙ a + a ∙ b - b ∙ a - b ∙ b =

a² + a b - a b - b² = a² - b²