Chapter 2, Problem 39RE

Suppose that $f\left(x\right)=M\sin x+N\cos x$ for some constants $M$ and $N$ . If $f\left(\pi /4\right)=3$ and $f^{\prime }\left(\pi /4\right)=1$ , find an equation for the tangent line to $y=f\left(x\right)$ at $x=3\pi /4$ .