(i) (7a − 9b) (7a − 9b) = (7a − 9b)^{2}

= (7a)^{2} − 2(7a)(9b) + (9b)^{2} [(a − b)^{2} = a^{2} − 2ab + b^{2}]

= 49a^{2} − 126ab + 81b^{2}

(ii) (− a + c) (− a + c) = (− a + c)2

= (− a)^{2} + 2(− a) (c) + (c)^{2} [(a + b)^{2} = a^{2} + 2ab + b^{2}]

= a^{2} − 2ac + c^{2}