p_(l) vector{s}=
=2*p_(l) vector{a} -4*p_(l)vector{b}+7*p_(l) vector{c}=
=2*| vector{a}|* cos альфа -4 | vector{b}|*cos бета +7*| vector{c}| *cos гамма=
=2*3*cos(-45 градусов)-4*2* cos(-60 градусов)+7*2*cos(120 градусов)=
=6*(sqrt(2))/2-8*(1/2)+14*(-1/2)=
=3sqrt(2)-11