x0 <- 0
y0 <- 0
r <- 5

f <- function(x,y)
  x^3+2*y^3-5*x*y+3*x^2+7*y

x <- seq(x0-r,x0+r,r/100)
y <-seq(y0-r,y0+r,r/100)
z <- outer(x,y,f)
t <- seq(0,2*pi,0.01)

contour(x,y,z,asp=1,nlevels=50)
lines(2*cos(t),3*sin(t),col="red")