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**quadrupole moment??**

The problem basically states that there are three point charges: q, -2q, q, placed along the z-axis at: a, 0, -a. The problem asks to find the monopole moment, dipole moment and quadrupole moment. I've been able to get the first to, but I have no idea how to go about getting the quadrupole moment. Tge prof never really explained it, so can someone please explain what the quadrupole moment matrix is....