Determine the first two terms, in ascending powers of $u$, of the binomial expansion of

$${\left(1+\frac{3}{4}u\right)}^{-\frac{1}{3}}$$

A signal processing engineer models the gain $G$ of a circuit at frequency offset $x$ using the formula:

$$G(x)=\cos (5x)+{\left(1+6x^2\right)}^{-\frac{1}{3}}$$

Hence, for small values of $x$, show that the gain can be approximated by

$$G(x)\approx A+Bx+C{x}^2$$

where $A$, $B$ and $C$ are constants to be found.