3 kg + 6000000 mg = ....... g

(3(1000)) g + (6000000 : 1000) = g
3000 g + 6000 g = 9000 g.

3 kg + 6.000.000 mg = g
3 kg × 1000 = 3000
6.000.000 ÷ 1000 = 6000
3000 g + 6000 g = 9000 g