documentclass{extarticle}
usepackage{amsmath,amssymb,physics}
begin{document}
    begin{align*}
        mu(x, y) = & phantom{{}+{}} sum_{k=1}^{2K} \ % \[-3.55em]
                    & + sum_{k=1}^{2K}
    end{align*}
end{document}