Answer
Rosemary had the correct equation because she took the 75+0.55b part of the equation out of the parentheses by using the "-" sign before it making 75 a negative and 0.55b a negative.
Work Step by Step
Rosemary did the problem correctly in using the original cost equation
$C=75+0.55b$
and incorporating it with the revenue equation
$R=0.95b$.
So, profit is P=R-C or $P=0.95b-(75+0.55b)$,
which then ends up being $P=0.95b-75-0.55b$,
then $P=-75+0.40b$.
