Lets make x and y the two numbers. The sum of the two numbers (x and y) is 9 means that they add to 9. So the first equation is
x+y = 9
Let's isolate y by subtracting x from both sides. That gives us
y = 9-x
The other bit of information given is "if one number is subtracted from the other, the result is -1", which means we have this other equation
x-y = -1
From here, substitute y for 9-x
x-y = -1
x-(y) = -1
x-(9-x)=-1
x-9+x=-1
2x-9 = -1
2x-9+9 = -1+9
2x = 8
2x/2 = 8/2
x = 4
If x = 4, then y is...
y = 9-x
y = 9-4
y = 5
Therefore, the two numbers are 4 and 5. They add to 9
4+5 = 9
and they subtract to -1
4-5 = -1
Final Answer: 4 and 5