dat<-data.frame(x=c(2,3,4,5,6),y=c(4,4,6,6,10))

x<-dat$x
y<-dat$y

min.RSS <- function(data, par) {
  with(data, sum((par[1] + par[2] * x - y)^2))
}

(result <- optim(par = c(0, 1), fn = min.RSS, data = dat))

plot(y ~ x, data = dat, main="Least square regression", xlim=c(0,6), ylim=c(0,10))
abline(a = result$par[1], b = result$par[2], col = "red")