Answer :
[tex]|a|= \left\{\begin{array}{ccc}a&if\ a\geq0\\-a&if\ a < 0\end{array}\right\\\\\\|91|\Rightarrow91 > 0\ therefore\ |91|=91\\\\Other\ examples:\\\\|4|\\4 > 0\Rightarrow|4|=4\\\\|-4|\\-4 < 0\Rightarrow|-4|=-(-4)=4[/tex]
