Answer:
3.3 inches.
Explanation:
We know that the scale of his building is 1 inch for every 400 ft.
And we also know that the actual length of the building is 1320 feet.
So, we can write the following proportion, with x being our unknown model length:

Note the the lengths line up horizontally. The numerator is all the model lengths, while the denominator is the actual lengths.
Solve for x. Cross multiply:

Divide both sides by 400:

Therefore, the length of the model is 3.3 inches.
And we're done!