(Calculus) When $\sin \alpha = \frac { 3 } { 4 }$, what is the value of $\cos 2 \alpha$? [3 points]\\ (1) $- \frac { 1 } { 32 }$\\ (2) $- \frac { 1 } { 16 }$\\ (3) $- \frac { 1 } { 8 }$\\ (4) $- \frac { 1 } { 4 }$\\ (5) $- \frac { 1 } { 2 }$