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 want to find the opposite side and we know the hypotenuse. Therefore, we should use sine.
sin(51) = BC / 58
BC = 58 x sin(51)
BC = 45.1 units
Hope this helps!