To find \(f'(x)\), we use the quotient rule for differentiation. The function \(f(x) = \frac{3}{2x+5}\) can be rewritten as \(3(2x+5)^{-1}\).

The derivative of \((2x+5)^{-1}\) is \(-1(2x+5)^{-2} \times 2\) using the chain rule.

Thus, \(f'(x) = 3 \times -1(2x+5)^{-2} \times 2 = -3(2x+5)^{-2} \times 2\).

Since \(f'(x)\) is negative for all \(x \neq -2.5\), the function \(f\) is decreasing.