Answer:
11/19
Step-by-step explanation:
Firstly, we need to know the total number of instructors. This is equal to 7+5+4+3 = 19 instructors.
The probability of selecting a science instructor is 4/19
The probability of selecting a math instructor is 7/19
The probability of selecting either a science or a maths instructor is 4/19 + 7/19 = 11/19
24a-18a+12a is equal to 18a
hence, divide it by 6a
so you get 3a :)