The Equation Gives the Position S = F(t) of a Body

The equation gives the position s = f(t) of a body moving on a coordinate line (s in meters, t in seconds) .
- $s = 12 \sin t - \cos t$
Find the body's velocity at time $t = \pi / 4 \mathrm { sec }$ .

A) $- \frac { 11 \sqrt { 2 } } { 2 } \mathrm {~m} / \mathrm { sec }$
B) $\frac { 13 \sqrt { 2 } } { 2 } \mathrm {~m} / \mathrm { sec }$
C) $\frac { 11 \sqrt { 2 } } { 2 } \mathrm {~m} / \mathrm { sec }$
D) $- \frac { 13 \sqrt { 2 } } { 2 } \mathrm {~m} / \mathrm { sec }$