For the function $f(x) = x + 1$,
$$\int _ { -1 } ^ { 1 } \{ f(x) \} ^ { 2 } dx = k \left( \int _ { -1 } ^ { 1 } f(x) dx \right) ^ { 2 }$$
what is the value of the constant $k$? [3 points]\\
(1) $\frac{1}{6}$\\
(2) $\frac{1}{3}$\\
(3) $\frac{1}{2}$\\
(4) $\frac{2}{3}$\\
(5) $\frac{5}{6}$