In: Electrical Engineering
Ans)
Given line voltages
  
phase voltages are




Given load current is


phasor diagram for this load is

from the phasor diagram above using inner triangle



is the reactance of the generator
=======
Now for full load 0.8 pf lagging 
full load current is


regulation at this current is


this answer is calculated using matlab hence it is accurate ,if you
use hand calculator neglecting some decimals you will get 13.5 % as
given in answer
Phasor diagram at this load is
