Question

Find g(x), where g(x) is the reflection across the x-axis of f(x)=x.

Answer 1

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.

Answer 2

G(x)=-x+0. Any equation reflected across the x axis will have the opposite x value as the original function. Hope this helps!

(f(x) in red, g(x) in blue)