Answer:
○

Explanation:
We are told that:
,
and told to find an expression for
.
In order to find
, we have to replace
in the definition of
with
:

Now we can simplify:
⇒
[Distributing 15 into the first brakets]
⇒

⇒
[Distributing the minus sign]
⇒
[Combining like terms]
⇒

Therefore, the first option is the correct one.