This is a question of direct proportionality. The mass of hemoglobin is expected to increase with increase in volume of the blood.
Therefore;
10 cm^3 of blood ---------- 1.2 g of hemoglobin
39 cm^3 of blood ---------- x g of hemoglobin
The,
x = (39*1.2)/10 = 4.68 g of hemoglobin.
That is, 39 cm^3 of blood contains 4.68 grams of hemoglobin.