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To solve this question you need to make another square that can have the tilted square inside it (shown in the diagram uploaded).

As you will be able to see, there are 4 identical triangles and a square inside the bigger square we made.

Now, we need to find the area of one of the triangles and multiply it by 4 (as they are identical).

Then we will need to find the area of the big square (side squared).

Finally , you need to subtract the area of the bigger square with area of triangles.

And you have your answer!