remember that anything raised to a power can be written as the number raised to a factor of that power, and then raised to the other factor:
3^6 = (3^3)^2
So anything raised to an EVEN power is a perfect square.
If you have several factors, each raised to an even power, they can all be rewritten as a perfect square. Remember:
(a^2)(b^2) = (a*b)^2
So if all 3 of your factors can be written like this, the whole thing can be written as a perfect square. But one of the numbers you gave was raised to an ODD power, meaning it can't be rewritten as a perfect square.