Ответ:

1)5х-(3+2х-2х²)=2х²-7х+11

5х-3-2х+2х²=2х²-7х+11

5х-3-2х+2х²-2х²+7х-11=0

10х-14=0

10х=14

х=14/10=7/5

2)12-(3х²+5х)+(-8х+3х²)=0

12-3х²-5х-8х+3х²=0

-13х+12=0

-13х=-12

х=12/13

3)(2у³+3у²-7)-(5+3у+у³)=3у²+3³-5у

2у³+3у²-7-5-3у-у³=3у²+у³-5у

2у³+3у²-7-5-3у-у³-3у²-у³+6у=0

3у-12=0

3у=12

у=12÷3

у=4