Sure! A rational expression is a fraction where the numerator and the denominator are both polynomials. An example of a relationship between \( x \) and \( y \) represented by a rational expression is:
\[ y = \frac{2x^2 + 3x + 1}{x - 4} \]
In this example, \( y \) is defined as a rational function of \( x \), with \( 2x^2 + 3x + 1 \) as the numerator and \( x - 4 \) as the denominator.