помогите решить образцы по алгебре, пожалуйста.

I)(4y-3y)/(2y+4)=y(4y-3)/(y+2)(y+2)
при y=-2: -2(4(-2)-3)/(-2+2)(-2+2); -2(-8-3)/00; -2(-11)/0; 22/0
Ответ: при y=(0;-2)

II)a)(7c-21d)/(2c-6d)=(7c-21d)(7c-21d)/2(c-3d)=(49c-147dc-147dc+441d)/2(c-3d)= (49c-294dc+441d)/2(c-3d)=7(c-3d)(c+3d)/2(c-3d)=7(c+3d)/2=(7c+21d)/2
b)(yyy-2yyy+yyy)/(7yyyy-7yyy)=y(yy-2yy+yy)/
/(y(7yyy-7yy)=(yy-2yy+yy)/(7yyy-7yyy)=(y-y)(y-y)/(7y(y-y )(y+y)=(y-y)/(7y(y+y)=(y-y)/(7yy+7y)

III)a)(x+12)/(16-x)+4/(4x+x)=(x+12)/(4-x)(4+x)+4/(x(4+x))=x(x+12)/x(4-x)(4+x)+4(4-x)/ (4+x)=(x(x+12)+4(4-x))/x(4-x)(4+x)=(x+12x+16-4x)/x(16-4x+4x-x)=(x+8x+16)/(x+16)=(x+4)(x+4)/(x+4)(x-4)=(x+4)/(x-4)
b)1/(ab+ab)+3/(a+b)=1/(ab(a+b))+3/(a-ab+b)(a+b)=(a-ab+b)/(ab(a+b))(a-ab+b)+3(ab)/(ab(a+b)(a-ab+b)=(a-ab+b+3ab)/ab(a+b)=(a+2ab+b)/(aab+abb)=(a+b)/(aba+abb)