Find the missing side length in the image below

Answer:
? = 5
Step-by-step explanation:
Recall: when 2 transversal lines cuts across 3 parallel lines, the parallel lines are divided proportionally by the transversals.
Therefore:
?/10 = 3/6
Cross multiply
?*6 = 3*10
?*6 = 30
Divide both sides by 6
? = 30/6
? = 5