Answer:
17 - 9 = 8
What you need to do is reverse the equation. What undos subtraction? Addition! so instead of x - 9 = 8, it turns into 9 + 8 = x. So whats the answer to 9 + 8? 17, so all in all x = 17 so to complete the equation, you need to plug in the value which makes the equation 17 - 9 = 8