[R] print formula on plot

[Thomas Steiner]

I estimate some parameters and I want to print them (pretty) on my plot:

# somehow estimated parameters
z<-c(1.543523e+00, 1.23453e+00, 3.454000e+00)

x<-seq(-1,1,length=100)
plot(x,z[3]*x^2+z[2]*x+z[3],type="l", main="My nice plot of the
estimated function")
zs<-format(z,digits=4,scientific=FALSE,trim=TRUE)

text(-0.9,7,expression(1.54*x^2))                   # is what I want,
but DYNAMIC
text(-0.9,6,expression(paste(zf[1],x^3)))         # not really
text(-0.9,5,expression(paste(toSting(zf[1]),x^3)))  # not really
#using z (double) instead of zf (string) does not help.

So my question is:
How do I evaluate zf[1] from the variable to it's (String) value? Here
it is used within an expression: this makes all the trouble.

Thanks for help, I tried 2 hrs now...
Thomas