What is the difference of a number s and 6 an expression

Which property is illustrated by the following statement? if hxa rgi, then rgi hxa.

Which is the graph of the piecewise function f(x)? f(x)=[tex]f(x) = \left \{ {{-x+1, x\leq 0 } \atop {x+1, x\ \textgreater \ 0}} \right.[/tex]

What are “like terms”? why can we only add like terms?