Let \(f(x) = \frac{15 - 6x}{(1 + 2x)(4 - x)}\).

(a) Express \(f(x)\) in partial fractions.

(b) Hence find \(\int_1^2 f(x) \, dx\), giving your answer in the form \(\ln \left( \frac{a}{b} \right)\), where \(a\) and \(b\) are integers.