Question

Which equation combines with the given equation to form a system of equations with the solution x = 3 and y = 9? x + 2y = 21

Answer 1

yes. x=3 and y =9 are the solutions.
Because if you change x=3 and y =9 ,you will have
3+2* 9 =21
3+18 =21
21 =21 (true)

Answer 2

x + 2y = 21

Explanation:

Even other options have not been given in this question, But we will prove that equation that combines to form a system of equations is x + 2y = 21

For this we will prove this equation true for x = 3 and y = 9

x + 2y = 21

3 + 2×9 = 21

3 + 18 = 21

21 = 21

Therefore, equation x + 2y = 21 combines to form a system of equations.