Money brought to school = \(\frac{3}{7}\) of weekly pocket money.<br>
At school, she spent $3 and had $6 left, so she brought $3 + $6 = $9.<br><br>
• \(\frac{3}{7}\) of weekly money = $9<br>
• Weekly pocket money = $9 &divide; \(\frac{3}{7}\) = $9 &times; \(\frac{7}{3}\)<br>
• = \(\frac{63}{3}\) = $21<br><br>
Her weekly pocket money was $21.<br>
Check: \(\frac{3}{7} \times 21 = 9\) (brought to school). Spent $3, left $6. ✓<br><br>
Final Answer: B: $21