thanks for all the help :)
A = ( [ 2a + 0 ] / 2, [2b + 0 ] / 2 ) = ( a, b )
C = ( [ 2c + 2c ] / 2 , [ 2d + 0 ] / 2 ) = (2c, d)
Slope AD = [ b - 0 ] / [ a - c] = b / [ a - c]
Slope BC = [ b + d - d ] / [ a + c - 2c ] = b / [ a - c ]
