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The expression shown is:

\[ \frac{4a^2 - 12ab + 9b^2}{4a^2 - 9b^2} \]

This can be simplified by factoring. The numerator and the denominator can be factored as follows:

1. Numerator:

   • \(4a^2 - 12ab + 9b^2\) is a perfect square trinomial and can be factored as \((2a - 3b)^2\).

2. Denominator:

   • \(4a^2 - 9b^2\) is a difference of squares and can be factored as \((2a - 3b)(2a + 3b)\).

So the expression simplifies to:

\[ \frac{(2a - 3b)^2}{(2a - 3b)(2a + 3b)} \]

Cancel out the common factor of \((2a - 3b)\):

\[ \frac{2a - 3b}{2a + 3b} \]