To substitute, we fist need one variable in terms of the other (plus a constant). Rearrange the first equation to get y in terms of x:

-x + y = 9

y = 9 + x

Substitute this value of y into the second equation:

x - y = -9

x - (9+x) = -9

x - 9 - x = -9

-9 = -9

This is true, for all values of (x,y) satisfying y = 9+x. So there are infinite solutions to this system of equations.

You can see this if you graph the lines - they're identical, i..e they perfectly overlap. So any point on the line is a solution to both equations = infinite solutions.