the diagonals of a rhombus intersect each other at the midpoints and they are also perpendicular . So at the smallest angle we have a right angled triangle with legs 6 and 3.5. Also this smallest angle (x) is bisected by the diagonal.
so we have:-
tan (x/2) = 3.5 / 6
x/2 = 30.256 degrees
x = 60.51 degrees to nearest hundderth
