Yup. First, in the Inspector Bar's Selector 3D Properties, enable "Link Selector Shell with Rotation Bar"; set both Extents CS and Coordinate System to "User CS." Select the rectangle, press "D" on the keyboard to tell Genie you want to relocate the Reference Point (the yellow bit), and Vertex-snap it to the left-hand end of the line you intend to make horizontal. While the rectangle is still selected, press and hold the Ctrl key and pick up the green "X" Rotation handle. Release the Ctrl key and Vertex-snap the rotation handle to the right-hand end of that same line. At this point, the Rot Z field of the Inspector Bar should display the angle of this line with respect to horizontal. Highlight the field; type a zero therein; press Enter. That oughta do it.