Let's equate the congruent sides.
Now, the segment EF is equal to the segment CD:
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First thing to do is to move the constant over to the other side of the equals sign. Then we will complete the square on the x terms. We will take half the linear term, square it, then add it to both sides. Our linear term is 4. Half of 4 is 2, and 2 squared is 4. 
. In the process of completing the square we have created a perfect square binomial on the left which is 
. A is our answer.