A plate in the shape of a parallelogram is acted upon by two couples. Determine (a) the mo...

A plate in the shape of a parallelogram is acted upon by two couples. Determine (a) the moment of the couple formed by the two 21-lb forces, (b) the perpendicular distance between the 12-lb forces if the resultant of the two couples is zero, (c) the value of a if the resultant couple is 72 lb • in. clockwise and d is 42 in.