It appears you are doing well. You have already gotten the eighth answer! If you need any other help you can ask me.
There are 4/7 boys, so there are 3/7 girls.
Let the total number of children be x.
4/7x = 8
1/7x = 8 ÷ 4
1/7x = 2
3/7x = 2 x 3
3/7x = 6
There are 6 girls.
The answer is = -3, hope this helps!