Question

How to write an equation for when a graph is decreasing

Answer 1

A decreasing graph can be obtained with a line that has negative slope. That is one of the simplest function you can have.

For example:

f(x) = - 3 x +2

or using the vertical variable "y" the form:

y = -x + 5

Either function will render a decreasing graph, as shown in the image below:

where the function y = - 3 x + 5 was plotted