What's Wrong with My Thevenin Equivalent Short-Circuit Current Calculation?

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Homework Statement

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Homework Equations

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Node-voltage method
mesh current method

The Attempt at a Solution

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My problem is with the short-circuit current.
There are three meshes when you join the nodes a and b.
Let i1 be the cw current for the bottom right mesh, i2 be the ccw current for the bottom left mesh, and i3 be the ccw current for the upper mesh.

Then I have

8(i1)+20(i1+i2)=0 for mesh 1
20(i1+i2)+5(i2-i3)+72=0 for mesh 2
5(i3-i2)+8(i3+i1)+12(i3)=0 for mesh 3

It turns out i1=6 when the system of linear equations is solved. But according to the answer it is the resistance not the current that has the magnitude of 6. What is wrong here?

 

[gneill]

Check your mesh 1 equation. You're missing the i3 contribution.