The blood glucose level of the person is 120 mg/dl. To express this in terms of mmol/l, the mass of glucose has to be converted from the units it is given in i.e. milligrams to the required units of millimoles; and the volume of blood has to be converted from deciliter to liter.
The molar mass of glucose is 180.16 g/mole. 120 mg of glucose is equivalent to 120/180.16 ~~ 0.667 moles. One deciliter is equivalent to 1/10 of a liter.
This gives the concentration of glucose in blood as 6.67 mmol/l