Please answer the following question

He mass of a single atom of carbon can be found by dividing the atomic mass (12.01 g) by 6.022 x 10^23. which is the mass of a single carbon atom, correctly written in scientific notation with the correct number of significant figures?

Which property was used to simplify the expression? 3c+9 +4c=3c+4c+9

240 seventh graders and two-thirds of the students participate in after-school activities how many students participate in after-school activities