heres the question

Using a version of the subtraction method, find the values of x, y, and z given the following equations:

x+y-z=6

2y+3z=-5

5z=-15

Now I figgured out the equation and have it all right, but the following question asks

How is it possible to have infanite solutions for this question. And I cant figgure out the answer to that. Any help is greatly appresheated.