Finding & graphing the intersection between two lines

intersection.R

f1 <- function(x) (3*x - 40) / 10
f2 <- function(x) (-20 - 2*x) / 2

# f1 <- function(x) (24000 - 200*x) / 350
# f2 <- function(x) (130 - x) / 2
x_limits  <- c(-10, 100)

########## Don't change anything below this line #########################
g <- function(x) f1(x) - f2(x)
y_range     <- range(0, f1(x_limits), f2(x_limits))
x_midpoint  <- mean(x_limits)
y_midpoint  <- mean(y_range)

plot(x=NA, y=NA, xlim = x_limits, ylim = y_range, xlab = "x", ylab = "y")
abline(v = 0, col = "gray70")
abline(h = 0, col = "gray70")
curve(f1, add = T, col = "blue")
curve(f2, add = T, col = "tomato")

root <- uniroot(g, lower = x_limits[1],  upper = x_limits[2])
x <- root$root
y <- f1(root$root)
root

abline(v = x, col = "green3", lty=2)
abline(h = y, col = "green3", lty=2)

if(x < x_midpoint) {
  text(labels=sprintf("y = %f\n", y), x=x_limits[2], y=y, adj = 1, col = "green3")
} else {
  text(labels=sprintf("y = %f\n", y), x=x_limits[1], y=y, adj = 0, col = "green3")
}  

if(y < y_midpoint) {
  text(labels=sprintf("x = %f", x), x=x, y=y_range[2], adj = 0, col = "green3")
} else {
  text(labels=sprintf("x = %f", x), x=x, y=y_range[1], adj = 0, col = "green3")
}