$\displaystyle \frac{\partial f_1}{\partial t}=D\frac{\partial^2 f_1}{\partial y^2}-C\bigg(\frac{\cosh(ky)}{\cosh(kh)}-\frac{\tanh(kh)}{kh}\bigg), \frac{\partial f_1}{\partial y}=0 \quad\mbox{at}\quad y=\pm h, f_1(0,y)=0$

How to solve the above equation. Please help me. $\displaystyle D, C, k$ are constants