To solve the proportion \( \frac{A}{B} = \frac{x}{750} \) where \( A = 500 \) and \( B = 750 \), we can set up the equation as follows:
\[ \frac{500}{750} = \frac{x}{750} \]
Cross-multiplying gives us:
\[ 500 \cdot 750 = 750 \cdot x \]
To isolate \( x \), we can simplify the left side:
\[ 500 = x \]
Thus, the solution for \( x \) is:
\[ x = 500 \]