Explanation:
since this is a symmetric figure, both diagonals have the same length.
let's calculate the one from the top left corner to the bottom right corner.
the length of the diagonal is simply the distance between these 2 points.
we get this via Pythagoras :
c² = a² + b²
c being the Hypotenuse (the side opposite of the 90° angle), a and b are the legs.
in our case, the distance is c, and the x-coordinate difference and the y-coordinate difference are a and b.
diagonal² = (-b - a)² + (c - 0)² = b² + 2ab + a² + c²
diagonal = sqrt(a² + b² + c² + 2ab)