Choice Dhoice B is the correct answer. To solve a division equation, we multiply both sides by the divisor.

1. Start with the equation: $\frac{x}{7} = 3$
2. Multiply both sides by 7: $x = 3 \times 7$
3. Calculate: $x = 21$
4. Verify: $\frac{21}{7} = 3$ ✓

💡 Key Pattern: For equations $\frac{x}{a} = b$, the solution is always $x = ab$ (multiply the numbers on the right side of the equation).

Choice Dhoice C is incorrect because this results from adding: $7 + 3 = 10$. Addition is not the inverse of division.

Choice Dhoice D is incorrect because this comes from subtracting: $7 - 3 = 4$. Subtraction doesn't undo division either.

Choice Dhoice A is incorrect because this results from dividing 3 by 7 instead of multiplying: $\frac{3}{7} \approx 0.43$. This applies division again, making the problem worse instead of solving it.