linear programing word problem

you are the assistant manager of an appliance store.Next month you will order two types of stereo systems.

i.model A: your cost is $300. your profit is $40

ii.model B: your cost is $400. your profit is $60

iii.you expect a profit of at least $4800.

iv.you expect to sell at least 100 units.

how many of each model should you order to minimize the cost?

**my equations might not be correct but is as far as i got**

(4800 >= 40a +60b)

y= 40a+60b 'y' equals the profit

maximum of each

4800>= 40a

4800>= 60b

my constraints ? ..... how do i graph it?....