Is it appropriate to use "it's my bread and butter" in formal english?
Solution 1:
Your English was generally good, but I have made some amendments in my editing which you might care to review. They mostly concern punctuation, but remember the definite article and prepositions are important too.
No, I see nothing wrong with using the 'bread and butter' metaphor. If I were the examiner I would be impressed that you were showing a grasp of everyday colloquial English.