An aIDitive fashion as predicted by the multiple-gene | Homework Helpers

The height of the seed head in wheat is determined by multiple genes that act in an aIDitive fashion as predicted by the multiple-gene hypothesis for quantitative inheritance. In one true-breeding variety of wheat, the seed head is only 9 inches above the ground. In another true-breeding variety, the seed head is 33 inches above the ground. Plants from the 9 inch variety were crossed to plants from the 33 inch variety. In all of the F1 progeny, the seed head was 21 inches above the ground. After self fertilization, the F1 plants produced a F2 population in which 9 inch and 33 inch plants each appeared with a frequency of 1/256.

If a 21 inch F1 plant were crossed to a 9 inch plant, how often would you expect 18 inch wheat to be found among the progeny?