(4k^2+7k)+(-k^2+3k-1)
=4k^2+7k-k^2+3k-1
=3k^2+10k-1
(5y+3x-15z^2)-(12y+7x+z^2)
=5y+3x-15z^2-12y-7x-z^2
=-7y-4x-16z^2