Torque of a square, very hard question?

Question

A rotational axis is directed perpendicular to the plane of a square and is located as shown in the drawing. Two forces, 1 and 2, are applied to diagonally opposite corners, and act along the sides of the square, first as shown in part a and then as shown in part b of the drawing. In each case the net torque produced by the forces is zero. The square is one meter on a side, and the magnitude of 2 is 3 times that of 1. Find the distances a and b that locate the axis. Note that a and b are not drawn to scale.

I'm not sure where to begin with this question. Can someone give me some tips? I'd greatly appreciate it.

Thanks.


Here is an image

http://www.webassign.net/CJ/p9-10.gif

Answer 1

Since torques (moments) cancel each other
M1+M2=0 and since M=r x F (x -implies vector cross product)
b x F1 - aF2=0 (minus sign for M2 since it is clockwise)
since F2=3F1 and radii are perpendicular to a respective force we can write

b F1 - 3a F1=0 from where
b - 3a=0 or
b=3a
We can say that so long as 0
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