It can be useful to know the prime factorization of the number:
[tex] 256 = 2^8 [/tex]
Then, remember that the n-th root of a number is the same as that number to the 1/n-th power:
[tex] \sqrt[4]{256} = 256^{\frac{1}{4}}[/tex]
Finally, you have to remember the rule for raising a power to another power, i.e. you multiply the exponents:
[tex] (a^b)^c = a^{bc} [/tex]
Put together everything we said so far to write
[tex] \sqrt[4]{256} = 256^{\frac{1}{4}} = (2^8)^{\frac{1}{4}} = 2^{\frac{8}{4}} = 2^2 = 4 [/tex]
