Pick any negative integer you want for x. Let's say we pick x = -10. Replace x with this value. So replace x with -10
x - y = -1
-10 - y = -1
Now isolate y. We do this in two steps. First we add 10 to both sides. Then we multiply both sides by -1
-10 - y = -1
-10 - y + 10 = -1+10
-y = 9
-1*(-y) = -1*(9)
y = -9
So one ordered pair is (x,y) = (-10,-9). There are infinitely many of these ordered pairs. Another ordered pair is (-11,-10) which is found following the same steps as shown above. The only thing that matter is that x-y is equal to -1. There are infinitely many ways to subtract two values to get -1.