If 14 of the 50 fish were trout, we can write this as a fraction; 14/50.
To get this into an easier-to-understand form, we can multiply both sides by 2, giving us a fraction of 28/100. This means that 28 out of 100, or 28% of fish, will be trouts, at least for a rough expectation.
From there, we can multiply 28%, the percentage of trout, by 7,500, the total amount of fish in the lake.
7,500 x 28% = 2,100
Therefore, we can roughly expect about 2,100 of the fish in the lake to be trout.
Hope this helps! :)