Let $a$ and $b$ be real constants such that the function $f$ defined by
$$f(x) = \begin{cases} x^2 + 3x + a, & x \leq 1 \\ bx + 2, & x > 1 \end{cases}$$
be differentiable on $R$. Then, the value of $\int_{-2}^{2} f(x)\, dx$ equals\\
(1) $\frac{15}{6}$\\
(2) $\frac{19}{6}$\\
(3) 21\\
(4) 17