Choice Ahoice C is the correct answer. When a number is being subtracted from the variable, we add to both sides to solve.

1. Start with the equation: $x - 7 = 15$
2. Identify the operation: 7 is being subtracted from $x$
3. Apply the inverse operation: Add 7 to both sides
4. Simplify: $x = 15 + 7 = 22$
5. Verify: $22 - 7 = 15$ ✓

💡 Remember: The inverse of subtraction is addition. To undo "minus 7", we add 7.

Choice A is incorrect because this results from subtracting 7 from 15 instead of adding: $15 - 7 = 8$. This applies the same operation instead of the inverse.

Choice C is incorrect because this is the negative of Choice A. Some students might calculate $15 - 7$ and then incorrectly apply a negative sign, getting $-8$.

Choice D is incorrect because this confuses the constant being subtracted (7) with the solution. The number 7 appears in the equation, but it's not the value of $x$.