Solution
For this case we see that b is one side of the parallelogram and c the height and we can find the area on this way:
A= b*c
We also see that a is the other side of the parallelogram and the corresponding height is d so then the area can be expressed as:
A= a*d
And since the area is the same we can do this:
bc = ad
And this equation must be true since the area of a parallelogram is the same
