Question within image.

1)a parallelogram is a quadrilateral with opposite sides being parallel so the lines AB must equal CD or otherwise it wouldn't be a parallelogram

2)for a quadrilateral to be proven to be a parallelogram the consecutive angles has to be supplementary which means they add up to 180 or for example you could slice the parallelogram in half down the middle you could piece the two consecutive angles together to make 180 degrees

go on the tube you might find some information there

5050

hope this !