So,
Melissa's rope is 84ft. long.
Each piece must be 7ft. long.
We divide the length of the rope by the required length of each piece.
Now we have a fraction: 84/7
Prime Factorization
Prime factored form of 84: 2*2*3*7
Prime factored form of 7: 7
Cross out 7's
2*2*3/1 or 12/1 or 12
Melissa can make 12 ropes.