I would write:
Please advise what you did to loose a C in your first line?
The original equation was;
a = 5b - 2c (3 + b) + C
In my first line of working out I got;
a = 5b - 6c - 2bc + c
in your first line of working out you got;
a = 5b - 6c -2bc
what did you do with the latter C ?
The final answer I am sure you had;
b = 4b - 5c / a - c