bearing in mind that in a parallelogram, both diagonals bisect each other, namely they cut each other in two equal halves, thus BE = DE. Check the picture below.
we can't use x = -3, because for DE that'd give use a 4(-3) = -12, which doesn't work for a segment with a length greater than 0.
well, since DE = 4(7) = BE, that means BD = DE + BE = 28 + 28 = 56.
