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Given the two similar triangles, how do i find the missing length? if a=4, b=5, and b'=7.5 find a'

What is the length and width of a 84.5 square foot rug? will mark

X/y + 4y/x divided by x/y - 2y/x explain as in depth as you can how you simplfied the fractions. i don't know where to start on this : /