(a-b)²
=a²-2ab+b²
=(a²+2ab+b²)-4ab
=(a+b)²-4ab
=(1/2)²-4×(-3/8)
=1/4+3/2
=7/4
a³b-2a²b²+ab³
=ab(a²-2ab+b²)
=-3/8×7/4
=-21/32