Dear Expert please answer;- 
                                                                                                     Here, if F is maximum, then why df / dm = 0 ??​

Dear student

It is a concept of maxima minima in mathematics

if you have a function 

y=f(x)

then,

any maximum or minimum value of y is found at points where dy/dx=0

eg, y=sin x is maximum at x=$\frac{\pi }{2}$

dy/dx= cosx

​dy/dx=0 

cosx=0

x=$\frac{\pi }{2}$

similarly in the question,

we have an equation of F in terms of m
so 

F is maximum at or minimum at values

dF/dm=0

here in question it is mentioned it is maximum. so we will get a max value


regards