Alright, first you have to realize that one pair of trousers (just one!) can create 4 different outfits by being worn with the four different coats. Each coat makes a new outfit with those trousers. The same thing can be said about each one of the trousers, so each trousers can combine with the coats to make 4 outfits. So basically that means that you will have 4 outfits per trouser multiplied by the 7 trousers. So you have 4x7 and that is equal to answer you got, 28
I drew a picture to explain the concept with smaller numbers. In the picture you can see that each pair of trousers creates 3 outfits by matching with the three shirts. Since this happens twice (there are two pairs of trousers) you get 3x2 and this equals 6, in my picture you could create 6 different outfits.
