Answer:
Length of each side = (4x-1)
Explanation:
You could use the Quadratic formula to solve this, but there is a bit of a logical/intuitive gap you have to jump over at the end to get the answer.
For a straightforward mathematical explanation of what the area is equal to, you need to complete the square. From there you acknowledge that a square's sides are equal to each other and then you'll have your lengths.
My work is in the attachment. Comment with any questions.