begin{equation}
    leftlbrace
        begin{aligned}
            k cdot E &= 0 \
            k cdot B &= 0
        end{aligned}
    right.
end{equation}

begin{subequations}
    begin{align}
        k cdot E &= 0 \
        k cdot B &= 0
    end{align}
end{subequations}

begin{equation}
    leftlbrace
        begin{subequations}
            begin{aligned}
                k cdot E &= 0 \
                k cdot B &= 0
            end{aligned}
        end{subequations}
    right.
end{equation}

/ k . E = 0     (10a)
    {
      k . D = 0     (10b)

/ k . E = 0
    {                 (11)
      k . D = 0