Explain with method pls..

See explanation below.

Explanation:

Given two blocks of masses M and m, And  a force F acting on the block of mass M pulling it outside.

The two masses system can be considered as a whole, With F  being the only external force acting on them.

Here the spring force has been ignored as while considering the two system as a whole, it will be accounted as an internal force for the system

Therefore, The acceleration of the system will be:

Now both the masses are moving with this same acceleration towards the direction of force .

Acceleration of mass m = a (as the system is moving as a whole)

Force on the mass m= mass times acceleration.