Choice Dhoice A is the correct answer.

1. Start with the equation: $7x - 2x = 30$
2. Combine like terms: $5x = 30$
3. Divide by 5: $x = 6$
4. Verify: $7(6) - 2(6) = 42 - 12 = 30$ ✓

Choice Dhoice C is incorrect because this results from dividing by 2: $30/2 = 15$.

Choice Dhoice C is incorrect because this results from dividing by 9 (adding terms instead of subtracting): $30/9 \approx 3.33$.

Choice Dhoice D is incorrect because this results from calculation error.