the question is this;

A charge Q is to be divided on two objects.What should be the values of the charge on the objects so that the force between the objects can be maximum?

here in the solution I am having problem.In solution it is written;

F=kQ(Q-q)/4*3.14*r^2 (coloumb.s law)

now let y=Q(Q-q)=Q^2-Qq.now for F to be maximum.this is the case when

dy/dq=0 i.e. Q-2q=0.

Now here I am having problem that why for F to be maximum dy/dq=0.please explain.