$(20\%)$ Problem $4$: Four point charges of equal magnitude $Q=75nC$ are placed on the corners of a rectangle of sides $D_1=19cm$ and $D_2=9cm.$ The charges on the left side of the rectangle are positive while the charges on the right side of the rectangle are negative. Use a coordinate system where the positive $y-$direction is up and the positive $x-$direction is to the right.