You can put this solution on YOUR website!
An even function is one where all of the exponents of x are even numbers.
An odd function is one where all of the exponents of x are odd numbers.
A function is neither if the exponents of x are both even and odd.
And remember that constants (numbers by themselves) are of even power, since 3 = 3*(x^0)!!
Therefore, your function:
f(x) = 5x^2 + x^4 is an even function, since 2 and 4 are both even (x^2 and x^4).