Answer: The tension in the string is 6 N
Step-by-step explanation:
Tension is defined as the pulling force that is applied on a string, chain etc.. Force is also defined as the mass multiplied by the acceleration of the object.
Mathematically,

where,
F = tension or force exerted in the string
m = mass of the cart pulling the string = 3.00 kg
a = acceleration of the string =

Putting values in above equation, we get:

Hence, the tension in the string is 6 N