This would be easier to explain if I had a paper and a pen, but I will give it a try :=)
Try drawing a picture of the problem and look at it while you read my explanation, it might help :=)
In a cross, the opposite angles will always be the same size, therefore, the opposing angle of the given is 72 degrees. Then we can use the knowledge that one of the unknown and one of the 72 will together be 180 degrees. Therefore, the other angle will be 180-72 = 108 and its opposite will be the same.
So the measures for the other three angles are: 108, 108 and 72.
