Ah ok The property is
"The Addition Property of Equality"
I can't see the following you miss of didn't pin it I think....
Well diameter will be the diagnol of that square..
let side of square be x, then diagonal will be root2 * x = diameter
so, raidus will be = dia/ 2
= (root2 * x ) /2
[ Answer ]

[ Explanation ]
4(y + 6) - 2(y - 2)
[Expand] 4(y + 6): 4y + 24
4y + 24 - 2(y - 2)
[Expand] -2(y - 2): -2y + 4
4y + 24 - 2y + 4
[Simplify] 4y + 24 - 2y + 4: 2y + 28
= 2y + 28
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