f(x) = 4x^4 - 4x^2

x0 = sqrt(21)/7

x1 = -sqrt(21)/7

i need to show, by hand, that you can calculate x1 from x0. i tried, but the answers didn't match up. exact square roots (no decimals) are to be used.

the equation i used was x1 = x0 - f(x0)/f '(x0), is that correct?