The image you have uploaded shows several mathematical expressions involving the order of operations (often referred to as PEMDAS: Parentheses, Exponents, Multiplication and Division (left to right), Addition and Subtraction (left to right)).

Let me analyze and solve the problems for you.

1. Problem 28:
   $3(8 - 4)$

   Solution:
   First, solve the parentheses:
   $8 - 4 = 4$
   Then multiply:
   $3 \times 4 = 12$
   Answer: $12$

2. Problem 29:
   $18 \div (6 - 1)$

   Solution:
   First, solve the parentheses:
   $6 - 1 = 5$
   Then divide:
   $18 \div 5 = 3.6$
   Answer: $3.6$

3. Problem 30:
   $3 \times [4 + (12 \div 4)]$

   Solution:
   First, solve the division inside the parentheses:
   $12 \div 4 = 3$
   Then solve the parentheses:
   $4 + 3 = 7$
   Finally, multiply:
   $3 \times 7 = 21$
   Answer: $21$

4. Problem 31:
   $\frac{9 - 7}{12 \div 3}$

   Solution:
   First, solve the subtraction in the numerator:
   $9 - 7 = 2$
   Then solve the division in the denominator:
   $12 \div 3 = 4$
   Now, divide the fraction:
   $\frac{2}{4} = 0.5$
   Answer: $0.5$

5. Problem 32:
   $(6 \div 2) \times 5$

   Solution:
   First, solve the division:
   $6 \div 2 = 3$
   Then multiply:
   $3 \times 5 = 15$
   Answer: $15$

Would you like more detailed explanations on any of these problems? Also, here are some related questions for further practice:

1. Solve $5 + 6 \times (3 - 1)$.
2. Simplify $4 \times (10 - 2) \div 2$.
3. Solve $(8 + 2) \div (3 \times 2)$.
4. Calculate $20 \div (2 \times 5) + 3$.
5. Simplify $\frac{18 + 6}{3}$.

Tip: Always start with operations inside parentheses and follow the PEMDAS rule (Parentheses, Exponents, Multiplication and Division, Addition and Subtraction).