Ответ: 8*Y^2
Решаем по действиям:
1) (3*X+Y)^2=9*X^2+6*X*Y+Y^2
2) 8*(9*X^2+6*X*Y+Y^2)=72*X^2+48*X*Y+8*Y^2
3) 12*(6*X+4*Y)=72*X+48*Y
4) (72*X+48*Y)*X=72*X^2+48*Y*X
5) 72*X^2+48*X*Y+8*Y^2-(72*X^2+48*Y*X)=72*X^2+48*X*Y+8*Y^2-72*X^2-48*Y*X
6) 72*X^2-72*X^2=0
7) 48*X*Y-48*Y*X=0
Решаем по шагам:
1) 8*(9*X^2+6*X*Y+Y^2)-12*X*(6*X+4*Y)
2) 72*X^2+48*X*Y+8*Y^2-12*X*(6*X+4*Y)
3) 72*X^2+48*X*Y+8*Y^2-(72*X+48*Y)*X
4) 72*X^2+48*X*Y+8*Y^2-(72*X^2+48*Y*X)
5) 72*X^2+48*X*Y+8*Y^2-72*X^2-48*Y*X
6) 48*X*Y+8*Y^2-48*Y*X
7) 8*Y^2