[R] Intercept of confidence interval with a constant

[Larry Howe]

Hello,

Is there a way for R to determine the point where a confidence interval equals 
a specified value? For example:

x = seq(1:5)
y = c(5, 5, 4, 4, 3)
lm = lm(y ~ x)
p = predict.lm(lm, interval="confidence")
matplot(p, type="b")
abline(h = 3)

I want to answer the question: "What is the value of x when the y-value of the 
lower confidence interval is equal to 3.0"? Visually, it is the place on the 
example where the abline intersects the lower confidence interval, or about 
4.2. Can R calculate this number for me?

I know that predict.lm will calculate a y-value from an x-value, but what I 
want is the opposite. I know the y-value, and I want to calculate the 
x-value.

Larry Howe