Since this is a right triangle, we can use one of the three main trigonometric functions: sine, cosine, or tangent.
Remember: SOH-CAH-TOA
Looking from the given angle, we know the opposite side and want to know the adjacent side. Therefore, we should use the tangent function.
tan(54) = 16/BC
BC = 16/tan(54)
BC = 11.62 units
Hope this helps!