In each case, the greatest common factor is the SMALLER of the two powers. Also, remember that xaxb = xa+b.
1. x3/2 - x1/2 = x1/2(x - 1)
2. (x+3)1/2 - (x+3)3/2 = (x+3)1/2[1 - (x+3)] = (x+3)1/2(-2 - x)
3. (x2 + 3)-2/3 + (x2+ 3)-5/3 = (x2+3)-5/3[(x2+3) + 1]
= (x2+3)-5/3(x2+4)