The sine rule is:

or

$$\small{\text{ Here we have: $ \boxed{ \dfrac{ \sin{(C)} }{c}= \dfrac{ \sin{(A)} }{a} } $}} \\\\ a\cdot \sin{(C)} = c\cdot \sin{(A)} \\\\ a = c\cdot \dfrac{\sin{(A)} }{ \sin{(C)} }\\\\ a = 12\cdot\dfrac{\sin{(80\ensurement{^{\circ}})} }{ \sin{(40\ensurement{^{\circ}})} }\\\\ a = 12\cdot\dfrac{ 0.98480775301}{ 0.64278760969}\\\\ a = 12\cdot 1.53208888624\\\\ a = 18.3850666349$$