What's (x+3 (x+5) it's really confusing me and i need some

Well when you see parentheses set up like these together that automatically means to multiply. The solution is:
Apply the FOIL method: (a+b)(c+d)=ac+ad+bc+bd
x2+5x+3x+15
 Simplify
x2+8x+15

It is neither a simple arithmetic nor geometric sequence, since it involves both a ratio and a difference.for  week "n", 40% of the previous week's bottles were sold, so 60% is what's left, so what's left = f(n-1)*(1-40%)=0.6f(n-1).48 bottles arrive, so we add 48 to what remains this week, givingf(n)=0.6*f(n-1)+48