[tex]|a|= \left\{\begin{array}{ccc}a&if&a\geq0\\-a&if&a \ \textless \ 0\end{array}\right\\\\examples\\3\geq 0\to|3|=3\\102\geq0\to|102|=102\\0\geq0\to|0|=0\\-4 \ \textless \ 0\to|-4|=-(-4)=4\\-105 \ \textless \ 0\to|-105|=105[/tex]
OK. Let's go to your question:
a) |-100| - 98 = 100 - 98 = 2
b) 5|2 - 8| = 5|-6| = 5 · 6 = 30
