Explanation: 
Another Explanation (5): ```html
Let the given equation of the straight line be \( ax + 3y - 7 = 0 \).
We can rewrite it as \( 3y = -ax + 7 \), or \( y = -\frac{a}{3}x + \frac{7}{3} \).
The slope of this line is \( m_1 = -\frac{a}{3} \).
Given that another straight line has a slope of \( m_2 = \frac{3}{2} \).
Since the two lines are perpendicular, the product of their slopes is -1.
Therefore, \( m_1 \cdot m_2 = -1 \).
\( \Rightarrow \left(-\frac{a}{3}\right) \cdot \left(\frac{3}{2}\right) = -1 \)
\( \Rightarrow -\frac{3a}{6} = -1 \)
\( \Rightarrow -\frac{a}{2} = -1 \)
\( \Rightarrow a = 2 \)
So, the value of \( a \) is 2.
Answer: 2.
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```
