If it takes 2 cups of m&m's to make 12 cookies. then you need to see how many times the original recipe will go into 84. Seeing that 84/12=7 you would need to multiply the original recipe by 7.
For example:
84/12=7 This is how many times the recipe is multiplied if you make 84 cookies.
2x7 is the amount of m&m's you would need to make the larger batch of cookies.
