Answer: 8.2
Step-by-step explanation:
As angles in a triangle add to 180 degrees,
[tex]\angle A=180^{\circ}-121^{\circ}-31^{\circ}=28^{\circ}[/tex]
So, by the Law of Sines,
[tex]\frac{a}{\sin A}=\frac{b}{\sin B}\\\\a=\frac{b \sin A}{\sin B}\\\\a=\frac{9 \sin 28^{\circ}}{\sin 31^{\circ}}\\\\a \approx \boxed{8.2}[/tex]
