work required to separate parallel plate capacitors

1. The problem statement, all variables and given/known data
A parallel plate capacitor with capacitance C is charged to a value q and then isolated. The separation between the plates is then tripled. What was the work required to separate the capacitors?

2. Relevant equations
U=1/2CV^2
Q=CV
C=ε0A/d

3. The attempt at a solution
W=ΔU=1/2C1V1^2-1/2C2V2^2
=1/2C*(Q/C)^2-1/2*(C/3)*(Q/C)^2
=Q^2/3C
I tried this but this wasn’t the correct answer and I can’t figure out why. Thank you in advance for the help

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