I am going through a book and having trouble with reproducing some results mentioned. The aim is to solve for $D_{s}$ from equation (1) below
$\int D_{s}(\vec{x}-\vec{a})D_{s}(\vec{y}-\vec{b})Q_{ss}(\vec{x}-\vec{y})d\vec{x}d\vec{y}=\sigma_{\mathrm{L}}^{2}\delta(\vec{a}-\vec{b})\ \ \ \ \ \ \ (1)$
The book says it can be done by writing $D_{s}$ and $Q_{ss}$ in terms of their fourier transforms as shown in equations (2) and (3) below.
$D_{s}(\vec{x}-\vec{a})=\frac{1}{4\pi^{2}}\int\exp(-i\vec{k}\cdot(\vec{x}-\vec{a}))\tilde{D}_{s}(\vec{k})d\vec{k}\ \ \ \ \ \ \ (2)$
$Q_{ss}(\vec{x}-\vec{y})=\frac{1}{4\pi^{2}}\int\exp(-i\vec{k}\cdot(\vec{x}-\vec{y}))\tilde{Q}_{ss}(\vec{k})d\vec{k}\ \ \ \ \ \ \ (3)$
Where $\tilde{D}_{s}$ and $\tilde{Q}_{ss}$ are the fourier transforms respectively.
The above then results in equation (4)
$|\tilde{D}_{s}(\vec{k})|^{2}\tilde{Q}_{ss}=\sigma_{\mathrm{L}}^{2} \ \ \ \ \ \ \ (4)$
I do not understand how to go from (1) from (4). I tried some simple substitutions but am unable to prove it.