Answer
a) 13.5
b) if $a=b$
Work Step by Step
a) The opposite of $-8.7$ is $+8.7$, so you add 4.8 and 8.7 to get the answer.
b) if $a=b$, then $a-b=0$, and $b-a=0$. Therefore, $a-b=b-a$
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