I don't see how (a^3-b^3)*(c/a-b) can simplify to (a^2+ab+b^2). One reason is that the first expression contains c, but the second one does not. On the other hand, (a^3-b^3)*c/(a-b) simplifies to (a^2+ab+b^2)*c, which can be checked by multiplying (a^2+ab+b^2) and (a-b).