15. Factorise:a2 + b2 - 2 (ab - ac + bc)
a2 + b2 - 2(ab - ac + bc)= a2 + b2 - 2ab + 2ac - 2bc= (a - b)2 + 2c(a - b)= (a - b) (a - b + 2c).
Factorise:(x - 3y)3 + (3y - 7z + (7z - x)3