.
Let A be the number of accountants and
let W be the number of waiters/waitresses.
The problem says
A - W = 9, (1)
A + W = 163. (2)
------------------------------ Add the two equations.
In this way you eliminate W:
2A = 9 + 163 = 172 ====> A = = 86.
Then from eq(2) W = 163-A = 163 - 86 = 77.
Answer. 86 accountants and 77 waiters/waitresses.
Check. Check it on your own - it is VERY USEFUL exercise for you, which will help you better to understand the problem.
P.S. I used the Elimination method solving the system of equations.