Which could be the length of the third side of a triangle with side lengths 3 ft and 9 ft? a 6 ft b 7 ft c 12 ft d 13 ft

It would be B. 7 feet. You can check and see what dimensions would make a triangle by adding two dimensions together and seeing if that sum is greater than the one dimension that you didn’t add.
Here’s and example:
3 plus 9 is 12 and 12 is greater than 7
7 plus 3 is 10 and 10 is greater than 9
7 plus 9 is 16 and 16 is greater than 3
If you can add any two side lengths together and get a sum that is greater than that third dimension than you can make a triangle with those dimensions.
I hope this helped! :-)