text: using b•0=0 (already proved this for you)\nb•0=b(1-1)=b+(-1)•b=0. (distributive property a(b+c)=ab+ac)\ntherefore -b+b+(-1)b=(-1)b=0+-b=-b (adding inverse, equivalent to cancellation property)\n\nNow using (-1)x=-x for all x:\n(-a)b=((-1)a)b=(-1)(ab)=-ab (associativity (ab)c=a(bc))