Simplify each expression in standard form. State the leading coefficient, if available. Then classify each polynomial according to its degree and number of terms. \text { 13. } 5+12 x^{2}+6 x-8 x^{2} \text { 14. } 7 x^{2} y-5 x^{2} y-3 x^{2} y \text { 15. } 3 x^{3} y^{2}-2 x^{3} y-x y^{2}+x^{3} y^{2}-6 x^{3} y \text { 16. }\left(6 x^{2}-8 x\right)+\left(11 x-6 x^{2}\right) \text { 17. }\left(-7 x^{3}-2 x+5\right)+\left(3 x^{3}-9 x\right) \text { 18. }(5 x+3)-\left(6 x^{3}-2 x^{2}\right) \text { 19. }\left(3 x^{2}-4 x\right)-\left(8+3 x^{2}-4 x\right)

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