Answer
$f=-1$
Work Step by Step
The two possibilities are $f+3=2f+4$
or $-f-3=2f+4$
The first equation, after subtracting f-4 from both sides,
gives f=-1 which when plugging back in gives |2|=|2|
The other equation gives $f=-7/3$ which when plugging back in gives,
|some number | = some negative number,
Which is extraneous since an absolute value cannot yield a negative number.
