Substitute $n=18$ into the function. You should get an expression like $p(18)=12,500-350\cdot 18$. What should you calculate first in that expression?

Find $350\cdot 18$ (it might help to think of $35\cdot 18$ and then multiply by 10). Then subtract that product from 12,500 to get the predicted number of fish.

In a Desmos expression line, type 12500-350*18 and look at the numerical result; that value is the predicted number of fish remaining after 18 months.

The function $p(n)=12,500-350n$ gives the number of fish $n$ months after the project begins.

For 18 months after the project begins, substitute $n=18$ into the function, so you want to find $p(18)$.

First, calculate how many fish are removed in 18 months at a rate of 350 fish per month:

So, a total of 6,300 fish are removed over 18 months.

Now subtract this total decrease from the initial 12,500 fish:

So, the predicted number of fish remaining after 18 months is 6,200, which corresponds to answer choice B.