If the polygons are similar, then we can set up a similarity ratio using sides AB and EF and any other 2 sides where both side lengths are given. That appears to be true for sides BC and FG. Our similarity ratio would be this then, going from the bigger polygon stuff on top and the smaller polygon stuff on bottom:
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If we cross multiply we have 8y = 36 and divide both sides by 8 to get that y = 4.5. There you go!