Existence and Uniqueness of Solution of Volterra Integral Equations

Question

Let $f(x),u(x),K(x)\in L^2(0,\pi)$, where $f(x),K(x)$ are known function. Find existence and uniqueness of solution of Volterra Integral Equation:

$$f(t)= u(t) - 2\int_{0}^{t} \int_{s}^{\pi} u(x)K(x-s)dxds, \hspace{2mm} t \in [0,\pi]. $$