In ΔABC, sin A = 1/4, sin B = 1/8, and b = 20. What is the length of a?
Do I apply the law of sines here?
Try the law of sines.
$\displaystyle \frac{sinA}{a}=\frac{sinB}{b}$
$\displaystyle a=\frac{bsinA}{sinB}$
The law of cosines is used when we have angle, side, angle. Like in the diagram: