Step 1. Get rid of the parentheses: XY – YZ – ZT – XT + ZY – ZT.

Step 2. Get rid of all the minus signs, using the fact that –AB = BA.

Step 3. Group the six vectors into three pairs and combine each pair into a single vector, using the fact that AB + BC = AC (you'll have to rearrange the order of the six vectors to get them to pair up properly).