If you mean (2y^3 -y +3) - (3y^2 -y -4) then
(2y^3 -y +3) - (3y^2 -y -4)
distribute the - into the parentheses on the right
2y^3 -y +3 -3y^2 +y +4
there's only one part with a y^3 and one part with a y^2, so there's nothing to combine with those
combine the parts with just y in them
2y^3 +3 -3y^2 +4
combine the parts without a y
2y^3 +7 -3y^2
the line above is correct, but for some reason we usually write the answer in order of highest exponents first, like this
2y^3 -3y^2 +7