Now we just need to calculate NDVI for each year as a measure of vegetated cover. It will make your life easier in the next step if you do them in the order specified below. You've done this a couple of times already, but if you forgot what to do click on the links for an animation showing the operations.
1. Open the Image Analysis window.
2. Select composite_april_2011.img and calculate NDVI by clicking the NDVI button in the Image Analysis window.
3. Calculate NDVI for composite_april_1996.img.
4. Calculate NDVI for composite_march_1984.img.
5. If you did the steps in order, you should see the three NDVI images in order from 1984 to 1996 to 2011 in the Image Analysis window. If the NDVI images are out of order, then drag them around in the Table of Contents.
Now on to Step 3 to make a color composite of the three NDVI layers.