The name of the shape is an octagon. Since there are 9 identical tiles and you have 72 sides, you would divide 72 by 9 and get 8. And shape that has 8 sides would be an octagon.
Sorry, but I don't really understand the question. Explain it more to me in the comments, and I'll try to give an answer.
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Length of the outer frame: 4+2x
width of the outer frame: 3+2x
area of the outframe: (4+2x)(3+2x)=15
expand the equation: 12+8x+6x+12x^2=15
combine like terms: 4x^2+14x-3=0
