R：在图表标题中显示计算

I have this code:

我有这个代码：

    z <- rnorm(20, mean = 5, sd = 1)
    hist(z, 
     main =substitute(paste("Title: " (hat(X), "=",x, sep=" ")), list(x=mean(z))), 
     xlab="z", xlim=c(2,8))
    z <- rnorm(20