You have one statement

"If you start with 1 male cannabis plant and 1 female Cannabis plant and allow them to have sex you create 100 new seeds from that single plant."

and one question.

"If you have 4 grow seasons per year ( 3 months) and each female plant produces 100 seeds with 50 females.... how long does it take before you have 1 Billion Cannabis plants?"

She answered the question correctly. It adds up to 50,000 years if you follow the question as stated.