Currently (i.e. 0 years) the population is 2.4 million,
In 35 years, the population will be 2.4 x 2 = 4.8 million.
Rate of growth = (4.8 - 2.4)/(35 - 0) = 2.4/35 = 0.06857
In 20 years, the population will be 2.24 + (20 - 1) x 0.06857 = 2.24 + (19 x 0.06857) = 2.24 + 1.303 = 3.54 million people.