Arrange the terms of the following algebraic expression in the descending order of their powers.

$-2{\mathrm{x}}^2{\mathrm{y}}^2+3{\mathrm{x}}^3{\mathrm{y}}^2+5\mathrm{xy}-4{\mathrm{x}}^3{\mathrm{y}}^3$

Add the following terms.$$

$13x, -27x$ and $-39x$

Add the following terms.

$19x,-55x$ and $\frac{2}{3}x$

(Mention your answer in fraction form)

Add the following terms.

 $7\mathrm{xyz}, -4\mathrm{xyz}$