Question

which triangle faction should be used to solve for the side BC if you know B and side AC

Answer 1

we have a right triangle, then we can use triangle fractions

to find CD with the angle B we use tangent because

$\tan (\alpha)=\frac{O}{A}$

where lapha is the reference angle(on this case B), O the opposite side from the triangle and A the adjacent side from the angle differente to the hypotenuse

$\tan (B)=\frac{5}{BC}$

and we can solve for BC

$BC=\frac{5}{\tan (B)}$