Answer :

Lin
Here is an example.
/ y=3x+2, x<2
| y=4x+7, [tex]2 \leq x \leq 6[/tex]
\ y=x-5   , x>6

you want find the value of y when x=4.5
Firstly find which domain/section this x belongs to,
clearly it belongs to the second domain. then replace the x in the second equation with 4.5, find y's value.
One thing you should know about piecewise functions is that for each x there will be only one corresponding y, vice versa.