you used \(\displaystyle y\) in the calculation \(\displaystyle z-y=-2\)

Here is another example

\(\displaystyle x=7;y=5;z=4;\)

\(\displaystyle k=1\) , \(\displaystyle a=1,2,3\) and \(\displaystyle b=3,2,1\)

\(\displaystyle k=2\) , \(\displaystyle a=3,4,5,6,7\) and \(\displaystyle b=5,4,3,2,1\)

\(\displaystyle k=3\) , \(\displaystyle a=7\) and \(\displaystyle b=5\)

for a total of \(\displaystyle 9\) pairs