How to find equations by looking into any graph please provi

How to find equations by looking into any graph? please provide explination in detailed , in english.

The below example, i really don\'t know how to find function from graph.

And other example:

Solution

15. Here we have three lines

For the first line the equation will be f(x)=-x -pi and the constraint is -pi<x<-pi/2

For the second line the equation will be f(x)=x and the constraint is -pi/2<x<pi/2

And for the third line the equation will be f(x)=x and the constraint is pi/2<x<pi