a)
[tex] \frac{a \sqrt{3} }{2} = \frac{12 \sqrt{3} }{2} = \boxed{ 6 \sqrt{3} cm}[/tex]
b)
[tex] \frac{a \sqrt{3} }{2} = 9 | \times 2 \\ a \sqrt{3} = 18 | \div \sqrt{3} \\ a = \frac{18}{ \sqrt{3} } \\ a = \frac{18 \sqrt{3} }{3} \\ \boxed{a = 6 \sqrt{3} cm}[/tex]
