Can you convert a decimal to a radical in simplest form?

Hello,

Can you help me find how to convert a decimal number to a radical (not rational) in simplest form? I need the answer with the check mark in it. ;^) For example, 3(check mark)5 = 6.70820394. But, how do you get that in reverse if you only know the decimal answer?

Otherwise, do you know of an online calculator or solver, or something like the TI-83 Graphing Calculator that would convert this?

Thanks,

Dan

PS

This seems to be essential for pre-calc, calc, and trig problems, but I haven't found an answer after several hours of searching on the Internet. Thanks for the help.