Write the function for each graph described below.
13. the graph of f(x) = x² reflected across the x-axis
14. the graph of f(x) = x² reflected across the y-axis
15. the graph of f(x) = x¹ translated 5 units up
16. the graph of f(x) = stretched horizontally by a factor of 3
17. the graph of f(x) = x³ translated 3 units down
18. the graph of f(x) = x² translated 4 units left