Initially you have 12+9=21 students
To choose the first one as a girl, you have 9 options out of 21, so the probability is 9/21
Next, to do this the second time, you have 8 girls left, out of 20 students, so 8/20
Again, we have 7 girls and 19 students, 7/19
And finally we are left with 6 girls and 18 students, so the probability of picking the last student as a girl is 6/18
To get the probability of the 1st and 2nd and 3rd and 4th student as a girl, focus on the AND
AND means multiplication, multiply the 4 probabilities and that's your answer.
If you have any more doubts feel free to message me.