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C. 18

Step-by-step explanation:

The two triangles are similar, because their angles are all congruent but their lengths are different. Set up a proportion and cross-multiply:

Make sure always to set these up with the numerators from the same triangle sides and the denominators from the other triangle, otherwise this won't work.

Now cross-multiply:

15x = 270

And solve:

270/15 = x

x = 18