Let's evaluate each expression given $x = 7$, $y = 3$, and $z = 9$.

13. $z + 2$ $9 + 2 = 11$

14. $5 + x$ $5 + 7 = 12$

15. $2 + 4z$ $2 + 4 \cdot 9 = 2 + 36 = 38$

16. $15 - 2x$ $15 - 2 \cdot 7 = 15 - 14 = 1$

17. $\frac{6y}{z}$ $\frac{6 \cdot 3}{9} = \frac{18}{9} = 2$

18. $\frac{9x}{y}$ $\frac{9 \cdot 7}{3} = \frac{63}{3} = 21$

19. $\frac{xy}{3} + 2$ $\frac{7 \cdot 3}{3} + 2 = 7 + 2 = 9$

20. $10 - \frac{xz}{9}$ $10 - \frac{7 \cdot 9}{9} = 10 - 7 = 3$

21. $4z - 3y$ $4 \cdot 9 - 3 \cdot 3 = 36 - 9 = 27$

22. $3x - 2y$ $3 \cdot 7 - 2 \cdot 3 = 21 - 6 = 15$

23. $2x + 3z + 5y$ $2 \cdot 7 + 3 \cdot 9 + 5 \cdot 3 = 14 + 27 + 15 = 56$

24. $5z - 3x - 2y$ $5 \cdot 9 - 3 \cdot 7 - 2 \cdot 3 = 45 - 21 - 6 = 18$

25. $7z - (y + x)$ $7 \cdot 9 - (3 + 7) = 63 - 10 = 53$

26. $(8y + 5) - 2z$ $(8 \cdot 3 + 5) - 2 \cdot 9 = (24 + 5) - 18 = 29 - 18 = 11$

27. $3y + (7z - 4x)$ $3 \cdot 3 + (7 \cdot 9 - 4 \cdot 7) = 9 + (63 - 28) = 9 + 35 = 44$

28. $6x - (z - 2y) + 15$ $6 \cdot 7 - (9 - 2 \cdot 3) + 15 = 42 - (9 - 6) + 15 = 42 - 3 + 15 = 54$

29. $2x + (4z - 13) - 5$ $2 \cdot 7 + (4 \cdot 9 - 13) - 5 = 14 + (36 - 13) - 5 = 14 + 23 - 5 = 32$

30. $(9 - 3y) + 4z - 5$ $(9 - 3 \cdot 3) + 4 \cdot 9 - 5 = (9 - 9) + 36 - 5 = 0 + 36 - 5 = 31$

Would you like more details or have any questions?

Here are 5 questions you might want to ask:

1. 如何求解二次方程？
2. 如何计算函数的导数？
3. 什么是三角函数的基本恒等式？
4. 如何计算矩形的面积？
5. 什么是对数的基本性质？

Tip: 熟悉常用的代数公式和计算方法，可以帮助你更快速地解决数学问题。