Why is this molecule chiral?

I don't understand why this molecule will rotate plane polarised light.

I look out for 4 different groups attached to the C atom (with CH3 group). That is the C with H and CH3 attached. I just need to decide between the remaining 2 groups.
At the 4 position (where the C=C bond is), I don't know how to assign as the carbons are at "equal intervals" between each other. I can't think of a better way to phrase this, I'm sorry!
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