WE have:

-a + (-(-a)) = 0 and also a + (-a) =0 and from these two equations by using the cancellation law : if a +b = a +c then b =c we get;

a = -(-a).

we also have:

(-a)(-b) = (-1)a(-b) = a((-1)(-b)) = a(-(-b)) = ab ,since (-1)x = -x.

And for the last one we have: =

-(a/b) = - = (-1)(a =