Seeing as they are similar shapes, you can divide the larger side length by the smaller one. In this case, 6.5 divided by 4.
6.4 ÷ 4 = 1.625.
Next, we write an equation using what we just learned. :)
13 = 1.625x
Divide both side by 1.625 to get 8 = x.
So the missing side length is 8.
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