Horizontal leg = 6 units
Vertical leg is 4 units
Lets Represent the diagonal as line AB where A and B are the endpoints of the line segment
The coordinates of point A is ( 12,4)
The coordinates of point B is ( 6,0)
Slope = ( Y2 - Y1 ) / (X2 -X1) = (0 - 4) / ( 6 - 12) = -4 / -6 = 2/3
or simply.
slope = rise / run = vertical leg/ horizontal leg = 4/6 = 2/3
ANSWER
Horizontal leg = 6 units
Vertical leg is 4 units
Slope = 2/3