first, let one of the groups equal "x students". The other group must then be "(2x+4) students" because it says in the problem that one group has 4 more than twice as much as the other group. the equation you can use to figure out what "x" equals is x+(2x+4)=28 because there are 28 students in total. By finding "x" which is equivalent to group 1, you can also find group 2 by plugging the number you got for x into the expression "2x+4".
x+2x+4=28 3x=24 2(8)+4=group 2
3x+4=28 x=8 (group 1 = 8 students) 20=group 2
3x+4=28