$a)\underline{18x^5}-\underline{\underline{6x^4}}+\underline{\underline{\underline{3x}}}-\underline{\underline{4x^4}}+\underline{2x^5}-\underline{\underline{\underline{3x}}}+8-\underline{10x^5}=$

$=10x^5-10x^4+8$

$b)\underline{-4x^{2}y}-\underline{\underline{8x{y}^2}}+\underline{\underline{\underline{2xy}}}-\underline{6x^{2}y}+\underline{\underline{9x{y}^2}}-8x^2+9y^2-\underline{\underline{xy}}=$

$=-10x^{2}y+x{y}^2+xy-8x^2+9y^2$

$c)\underline{\frac{1}{2}km}-\underline{\underline{\frac{3}{4}{m}^2}}-\underline{2,5km}+\underline{\underline{0,75m^2}}-\underline{\underline{\underline{9k^2}}}+6m-\underline{\underline{\underline{3k^2}}}=$

$=-2km-12k^2+6m$

$d)\underline{-pr}+\underline{\underline{pq}}-\underline{\underline{\underline{pqr}}}-\underline{pr}-\underline{\underline{\underline{\underline{qr}}}}+\underline{\underline{pq}}-\underline{\underline{\underline{pqr}}}+\underline{\underline{\underline{\underline{qr}}}}=$

$=-2pr+2pq-2pqr$

$e)\underline{9x^{3}y}+\underline{\underline{4x{y}^2}}-\underline{5x^{3}y}+\underline{\underline{7x{y}^2}}-\underline{4x^{3}y}-\underline{\underline{11x{y}^2}}=0$