The key to solve this problem is to note that opposite of a paralelogram are equal!
The sides that have a red line over them are equal (the up and down sides).
We see that the point B has coordinates: (z , y)
And because of the property of paralelograms, the point C is x units to the right of the point B, so its coordinate in the x-axis is equal to: z+x , and the coordinate in the y-axis is the same