Find the missing side, given two triangles

The figure shows ∆ABC ∼ ∆DEF with side lengths as indicated.

What is the value of x?
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∆ABC ∼ ∆DEF states that the two triangles are similar.  
​In similar triangles, the corresponding sides are in proportion.  So, set up a proportion to solve for x.

5 in ∆DEF matches up with x in ∆ABC
9 in ∆DEF matches up with 27 in ∆ABC

 5          9
---   =   ----   {set up a proportion of corresponding sides}
 x         27

9x = 135   {cross-multiplied}
x = 15   {divided each side by 9}

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