i want to know how to calculate upper and lower bounds, is there any particualr formula we have to use?

the question is...

we would like to pack as many circles of diameter d as possible into another larger circle of diameter D. we donate by N(d,D) such number

a) give an upper bound for N(d,D)
b) give in details how we can calculate a lower bound for N(d,D). find an explicit value for N when d=6mm and D=48mm.

someone please help!!!! i've had a look at some websites but im having problems understanding the concept, pleas explain!