According to the equation of known straight lines, let x=0 and y=0 respectively, and find out the two intersections with the coordinate axis, so that two straight lines can be drawn in the coordinate system. Their intersection point is a, and then according to the coordinates of the midpoint, the coordinates of point C are obtained. Finally, according to the parallelism of parallelogram, draw two other straight lines passing through point C to form parallelogram.

3x-y+4=0, then y=3x+4, so the slope is 3.

X+y- 1=0, then y=-x+ 1, so the slope is-1.