2...and...3}.---two pairs of opposite sides are parallel and equal
4.}--opposite angles are equal
5…}---diagonals bisect each other
none. I has no solutions (in all x, y and z) because in case x+y+z is 3 it can never be 4. So, it's imposible to find any (x,y,z) so that they satisfy both equations.
To find the slope, convert the equation 21x - 3y + 18 = 0 to slope-intercept form (y = mx + b, where m is the slope and b is the y-intercept). To do so, first subtract 18 and 21x from both sides to get -3y = -21x - 18. Then divide both sides by -3 to get y = 7x + 6. Therefore the slope is 7.
