Answer
The force on the block from the rope is $~~\frac{F~M}{M+m}$
Work Step by Step
We can find the acceleration of the rope and block:
$F = (M+m)~a$
$a = \frac{F}{M+m}$
We can find $F_r$, the force on the block from the rope:
$F_r = M~a$
$F_r = (M)~(\frac{F}{M+m})$
$F_r = \frac{F~M}{M+m}$
The force on the block from the rope is $~~\frac{F~M}{M+m}$
