prove -x - y = - y - x, for all integers x and y and does part (a) imply that subtraction is commutative
we know that
addition is commutative
multiplication is distributive over addition
so,
-x-y = -(x+y) = -(y+x) = -y-x
subtraction is not commutative, since
x-y = -(y-x) = -y+x
a single counterexample sufficeth:
3-4 = -1
4-3 = +1