What a resultant is
Because every force applied to the motor car was applied in the same direction, what happens is that all the pushing forces add together to make one large single force.
Resultant force
When more than one force is applied, the single force that produces the result caused by all of those forces together is called their resultant.
How to read the definition
The important word is result. A resultant is not a new force added on top of the others — it is the single force that could be put in place of all of them. If instead of the three people there had been one immensely strong person applying that much force in the same direction, the car would have moved in exactly the same way.
A resultant has a size and a direction
Force is a vector, so a resultant is a vector too. "The resultant is 14 N" is half an answer — you have to say in which direction as well. Keep that in mind for the whole chapter.