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 hypotenuse. Therefore, we should use the sine function.
sin(54) = 16/AB
AB = 16/sin(54)
AB = 19.78 units
Hope this helps!