You need to consider the carbon atoms on each side of the double bond separately.
Start with Carbon 2, the one that has the Cl group and the COOH group attached to it. Apply the priority rules (based on atomic number, not mass) to it. Cl has the highest priority.
Now look at Carbon 3, the one with the methyl group and hydrogen attached to it. Apply the priority rules to it. The methyl group has highest priority.
Now, looking at the double bond as a whole, the highest priority groups are both on the same side of the bond. Therefore, this is a Z isomer.
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