Please see FAQ: What's a reproducible example (`reprex`) and how do I create one?, and why it's important.
Fortunately, your question is simple enough not to require one to understand the issue
plot(mtcars$wt,mtcars$mpg)
Created on 2020-02-17 by the reprex package (v0.3.0)
This plot shows f(x) = y for x =wt and y =mpg. This is mileage (miles per gallon of fuel) against weight (in pounds), the x axis and the y axis, respectively.
What do you observe about the position of the mpg points as wt increases? Does it change? If so, how?