Переход к полярным координатам
x=rcos phi
y=rsin phi
0 < r < 1
0 < phi < 2Pi
m= ∫^(1) _(0)dr ∫ ^(2Pi)_(0)(3-rcos phi -rsin phi )d phi =
= ∫^(1) _(0)(3 phi -r*sin phi +rcos phi )|^(2Pi)_(0) dr
=∫^(1) _(0)(6Pi-r*(0-0)+r(1-1))dr=
=∫^(1) _(0)(6Pi)dr=(6Pir)|^(1)_(0)=6Pi