With a B driver's licence you may drive a car with a total weight of 3,500 kg and tow a light trailer with a total weight 750 kg (3,500+750=4,250).
However, if the trailer's total weight exceeds 750 kg and the car and trailer's combined total weight exceeds 3,500 kg, a B class driver's licence is not sufficient for the vehicle combination.
Example: If the trailer's total weight is 850 kg and the car's total weight is 2,750 kg, a B class driver's licence is not sufficient. This is because the total weight of the trailer exceeds 750 kg and the combined total weight exceeds 3,500 kg.
Keep in mind that both the car and trailer can have limitations regarding total weight and maximum load. Details of this can be found in part 1 of each vehicle's registration certificate.
Even if you have the correct driver's licence, it does not automatically mean that your car can tow the trailer or that the trailer can handle the load.