What is the line of symmetry for `f(x) = x^4`?

Related questions

• What functions have symmetric graphs?
• What are some examples of a symmetric function?
• What is a line of symmetry?
• What is rotational symmetry?
• Is the function `f(x) = x^2` symmetric with respect to the y-axis?
• Is the function `f(x) = x^2` symmetric with respect to the x-axis?
• Is the function `f(x) = x^3` symmetric with respect to the y-axis?
• Is the graph of the function `f(x) = 2^x` symmetric?
• Is `f(x)=x^2+sin x` an even or odd function?
• Is the product of an odd function and an even function odd or even?