12т 34кг-3456кг

To subtract 3456 kg from 34 kg, you would subtract the kilograms first and then the tonnes:

34 kg - 3456 kg = -3422 kg

To express the answer as tonnes, you need to convert the kilograms to tonnes by dividing by 1000:

-3422 kg ÷ 1000 = -3.422 tonnes

So, 12 tonnes minus 3456 kg is equal to -3.422 tonnes.