I encourage you to draw in the diagonal. The diagonal of a square will create a triangle with the sides of the square. Specifically, this is a right triangle as the angles of a square are right angles by definition.
Use the Pythagorean theorem: a^2 + b^2 = c^2. In this case, sides a and b are each 7sqrt(3), and c is the diagonal of the square. a^2 + b^2 = c^2 (7sqrt3)^2 + (7sqrt3)^2 = c^2 294 = c^2 c = sqrt(294) = 7sqrt(6)
