For each parallelogram, its area is the product of the lengths of any two its adjacent sides

by the sine of the angle between them


    Area = .


For a rhombus, this formula takes the form


    Area = ,


where "a" is the length of any side and  is any of the four angles of the rhombus.


Thus in our case


    40 = ,    =  = ,


and an acute angle of the rhombus is


     =  = arcsin(0.17778) = 0.17873 radians = 10.25 degrees (approximately).    ANSWER