Simplify by adding or subtracting like terms:

$4y-4y$

Simplify by adding or subtracting like terms:

$9x-10x$

Simplify by adding or subtracting like terms:

$y-4y$

Simplify by adding or subtracting like terms:

$5x-x$

Simplify by adding or subtracting like terms:

$9xy-2xy$

Simplify by adding or subtracting like terms:

$6pq-2qp$

Simplify by adding or subtracting like terms:

$14xyz-xyz$

Simplify by adding or subtracting like terms:

$4x^2-2x^2$

Simplify by adding or subtracting like terms:

$9y^2-4y^2$