Can someone help me find answer.

A coral reef grew 7 meters.

Step-by-step explanation:

The prefix "milli" before any standard  unit such as meters, grams, or seconds and etc., It means it takes 1000 milli units to make 1 standard unit. Example:

1 meter= 1 millimeters.

To solve this problem you need to cross multiply:

7000 millimeters              1 meter    

                       multiply                 = 7 meters

1 month              1000 millimeters

Units that are the same and diagonal from each other cancel out.  

So the answer is: Last month, A coral reef grew 7 meters.