Variation of generator working frequency with load
I think I see how the answer may be 49.917 Hz. 

Under no load conditions the output frequency is 52.5 Hz and under a full load of 15 MW it is 50 Hz. However the actual load placed on the generator is 12 MW + 3.5 MW hence the load is 15.5 MW. This means the output frequency is slightly lower than 50 Hz.

Each MW lowers the frequency by 0.16666 Hz hence, starting at 52.5 Hz the frequency lowers by 15.5 x 0.166666 Hz = 2.583333 Hz hence 52.5 Hz lowers to 49.916666 Hz.

Ignore my comments about the power lowering with frequency - I was just trying to get my head around the questions subtleties. It's unclear why you began with 52.5 Hz and dropped this in favour of 52 Hz as the open load output frequency.

I also think there may be an anomaly in the question regards the three 4 MW loads being fed at 50 Hz - this contradicts the 5% droop that I take to mean the output frequency will be higher by 5% at no-load compared to 15 MW full-load.