Find a square square pyramidal number.
Find a number which which is both square and square pyramidal.
Apart from 0 and 1, which may be considred trivial, the only solution is 4900, which is the sum of the first 24 positive squares. (This solution is asserted here on proofwiki to be unique, without a proof.)
I hope the metapuzzle is not too hard. The solution can be found by an obvious but tedious search.
This is one of several pages on puzzles and metapuzzles.