Since the figure is a rhombus, the diagonals cut each other at right-angles and bisect each other, namely cut their vertices in equal halves.
what the dickens does all that mean? well, the angles at vertex C and vertex A are all twins, and therefore, ∡BCA = x, therefore
what the dickens does all that mean? well, the angles at vertex C and vertex A are all twins, and therefore, ∡BCA = x, therefore