Let "x" represent the number of dozens sold, if each dozen is sold for $3, then "3x" represents the amount earned after selling "x" dozens of cookies.
You need to raise $1000 by selling cookies, then:

Divide both sides by 3

You need to sell approximately 334 dozens of cookies to raise $1000.
Multiply the result by 12 to calculate the number of cookies:

You have to sell 4000 cookies.