For each binary operation * defined, determine whether * is commutative and / or associative.

(a) on Z, define * by a*b = a-b

(b) on Q, define * by a*b=ab + 1

(c) on Z+, define * by a*b= 2^ab

The answers stated in my notes are

(a) not comm and not associative

(b) comm, not associative

(c) comm, not associative

Can someone care to explain to me the method to deduce the answers?

Thanks!