It has been found that two equal and opposite forces act on a moving body. How will its motion be affected? State the reason.

Hint: First law of motion says that everybody will be at rest or in the state of uniform motion in a straight line until it is compelled to vary its state by the action of an external force. This is the basic definition for the quantity called inertia.

Complete answer:
The first law of motion says that everybody will be at rest or in the state of uniform motion in a straight line until it is compelled to vary its state by the action of an external force. This is the basic definition for the quantity called inertia. The major point here is that when there is no resultant force which is caused by an unbalanced force experienced on a body that is if all the external forces will be cancelled out, then the object will be remaining at a fixed velocity. When that velocity is found to be zero, then the object will stay at rest. Whenever an extra external force is applied, the velocity will vary because of the force. The amount of the variation in velocity is indicated by Newton's second law of motion. So here, as the forces acting are equal as well as opposite, the resultant force acting will be zero. Therefore the body will remain at the initial state itself. That is the body will be in the same state of motion with similar velocity. Hence the question has been answered.

Note:
There are so many other examples of Newton's first law which includes aerodynamics also. The movement of an airplane if the pilot varies the throttle setting of the engine is explained by the first law. The movement of a ball which is coming down through the atmosphere or a rocket which is being launched up into the atmosphere are the common examples for Newton's first law of motion.