1. **Problem:** Find the unknown value □ in the proportion $3 : 4 = 6 : \square$. 2. **Understanding Proportions:** A proportion states that two ratios are equal. The formula for proportions is $$\frac{a}{b} = \frac{c}{d}$$ where $a$, $b$, $c$, and $d$ are numbers and $b, d \neq 0$. To find the unknown, cross-multiply and solve for the unknown. 3. **Apply the formula:** Given $3 : 4 = 6 : \square$, rewrite as $$\frac{3}{4} = \frac{6}{\square}$$. Cross-multiply: $$3 \times \square = 4 \times 6$$. 4. **Calculate:** $$3\square = 24$$. Divide both sides by 3: $$\square = \frac{24}{3} = 8$$. 5. **Answer:** The unknown value is $\boxed{8}$. This means the proportion $3 : 4 = 6 : 8$ holds true because both ratios equal $0.75$.