So for this problem you need to find the length of a side of the square. To do this you can use Pythagorean theorem to find the length of the right triangle with the side of the square as it’s hypotenuse.
The Pythagorean theorem is:
a^2 + b^2 = c^2
a = 1 and b = 3
1^2 + 3^2 = c^2
10 = c^2
square root of 10 = c
Now that we know the length of the side, we can find the area of the square. Area of a square = side^2, so:
Area = (square root of 10)^2 = 10
The area of the square is 10!