24? I am not entirely sure but I think a4=24
Reason: if a1 equals 6 then I would like to think you would multiply 6 by 4 to get the value of a4 (I haven’t done this type of math in a while)
Answer:

Step-by-step explanation:
That would be a divide by 5: 
2.18
Mark brainliest please
Hope this helps