(reading scales) what is the halfway point between 1200 and 1500

Adaylily farm sells a portion of their daylilies and allows a portion to grow and divide. the recursive formula an=1.5(an-1)-100 represents the number of daylilies, a, after n years. after the fifth year, the farmers estimate they have 2225 daylilies. how many daylilies were on the farm after the first year?

I’m stuck on this equation, anyone got the answer?

Which property was used to simplify the expression? 3c+9 +4c=3c+4c+9