Look at each term. How does the next one change? what is added?
The answer is 6, because there is an increase of 6 for each term
Since ABCD is a parallelogram, the opposite sides will be parallel and equal,

Consider that AC acts as a transversal to the parallel lines AB and CD, so we can write,

So by the ASA criteria, the triangle AED is congruent to the triangle CEB,
Then the corresponding parts of the triangles will be equal,

Hence Proved.