Преобразуйте в дробь \(\frac{7a}{a^2-9b^2}\cdot\frac{4a+12b}{28ab}\) .

• \(\frac{1}{ab+3b^2}\)
• \(\frac{1}{ab-3b^2}\)
• \(\frac{13a}{a^2-9b^2}\)
• \(\frac{4a+12b}{a-9b^3}\)