What is the greatest common factor of 84 and 210

I’m really confused and need to learn this

Julie begins counting backwards from 1000 by 2’s and at the same time tony begins counting forward from 100 by 3’s.   if they count at the same rate, what number will they say at the same time?

The height of a box can be found by dividing its volume by the area of its base,bottom.what is the height of a box that has a volume of 26.35 cubic centimeter and a base area of 4 1/4 square centimeter?