I wonder if you mean to write [tex]\sqrt[3]{256}[/tex] in place of [tex]3\sqrt{256}[/tex]...
If you meant what you wrote, then we have
[tex]\sqrt{256}=\sqrt{16^2}=16[/tex]
[tex]3\sqrt{256}=3\cdot16=48[/tex]
If you meant to write [tex]\sqrt[3]{256}[/tex] (the cube root of 256), then we could go on to have
[tex]\sqrt[3]{256}=\sqrt[3]{16^2}=\sqrt[3]{(4^2)^2}=\sqrt[3]{4^4}=\sqrt[3]{4^3\cdot4}=4\sqrt[3]4[/tex]
