Hey
> Diagonals of a Rhombus Bisects the Angles at the Vertex
=> For a vertice holding an angle 120°, the Bisected Angle = 60°
Further, the Angle at the Second Vertice = 60°
Hence, we have an Equilateral Triangle with two sides as 10cm and the third - a diagonal
=> By definition of an Equilateral Triangle, Diagonal = 10cm