Answer:
Infinite solutions
Step-by-step explanation:
x + y = 10
y = -x + 10
We'll choose one equation to use, x + y = 10
Substitute y with -x + 10
x - x + 10 = 10
x cancels each other out
10 = 10, which would also mean 0 = 0
This means the answer will validate with any number
