TI89Ti: Inaccurate intercepts

f(x)=$\displaystyle \sqrt{3x+4}$ and g(x)=$\displaystyle x^3$.

(fog)(x)=(gof)(x)

$\displaystyle \sqrt{3x^3+4}$=$\displaystyle (\sqrt{3x+4})^3$

If I plot $\displaystyle \sqrt{3x^3+4}$-$\displaystyle (\sqrt{3x+4})^3$=0,

and do a F5,2 (Zero), I get xc=-962025 and yc=-.02286

In fact, if I plot another graph, y2=0, and find the intersection, I get the correct answer (-1,0)

Why is there an inaccuracy in the Zero function?

Besides, my TI89Ti takes around 4 seconds to plot this graph. How can I speed up the graphing? I have disabled pretty-print, set the coordinates format to rectangular (in polar form, I noticed, the calc takes more time), disabled Discontinuity Detection (F3,Graph->F1,Tools,9:Format).

Any ideas?