What is the greatest common factor for 12 and 20 and 36

Well I don't know !
Let's find it together:

The factors of  12  are  1,  2,  3,  4,  6,  and  12 .

The factors of  20  are  1,  2,  4,  5,  10,  and  20 .

The factors of  36  are  1,  2,  3,  4,  6,  9,  12,  18,  and  36 .

The common factors of  12,  20,  and  36 (the ones that
show up on all three lists) are ...

                       1,  2, and  4 . 

The greatest one of these is  4 .

Its 3 and 4 i checked on class

The surface area would be 576 because you multiply all the numbers then multiply that number by 6