What statement makes the open sentence 7x-7=2x+8 true?

You can start by subtracting the 8 on both sides or adding the 7. I will add the 7 to make it positive. Then, subtract the 2x from both sides. You get 5x = 15. Divide the 5 from both sides, and you get x = 3. So, when x = 3, the sentence is true.