First, find all the vertices.

A(7,2)

B(7,4)

C(1,4)

D(1,2)

The diagonal |BD| has slope Dy/Dx.

m = Dy/Dx = (2-4)/(1-7) = 1/3

General line formula:

y = mx + C

y = 1/3x + C

We know that the line will pass through B(7,4) and D(1,2).

y = 1/3x + C

4 = 1/3 7 + C

C = 5/3

So the equation becomes,

y = 1/3x + 5/3

3y = x + 5

-x + 3y = 5

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Edit: Aaaah the question was about a square, I thought it was a rectangle :S

I don't have time to solve it at the moment, I think I can help you later..