Раскрыть скобки 7х-(5х-(3х+у))
7х-(5х-(3х+у)=7х-5х+(3х+у)=7х-5х+3х+у=5х+у
7х-(5х-(3х+у))=7х-(5х-3х-у)=7х-5х+3х+у=5х+у