Hi there! The answer is k = 16
To find our answer we only need to collect the terms. In other words: we need to subtract all the values of k.
Example:

If you don't get this, you can try to understand it by using a story: in a cage, there are 18 koalas, 11 of them leave. How many are left?
18 koalas - 11 koalas = 7 koalas
18 k - 11 k = 7 k
Now we keep doing this again and again to find our final answer:

~Hope this helps you!