Find the diagonal of a square whose sides are of the given measure.
7 square root of 3

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)