Find g(x), where g(x) is the reflection across the x-axis of f(x)=x. write your answer in the form mx+b, where m and b are integers.

The graph of f(x) = x in a straight line passing through the origin (0,0) and  at an angle of 45 degrees to the x axis. It increases from left to right.

Reflection in the x axis  will convert all positive values of f(x) to negative values of f(x)  and vice versa.  

g(x)  = - x     and is a striaght line passing through origin  at angle of 45 degrees but the slope descends from left to right.