{${a^{n} b^{n+k}}$ } U {${a^{n+k} b^{n}}$} => {${a^{m} b^{m}}$} where m=n+k or n+0,
now n>=0 and k>=1 or 3 becomes K>=1 then
possible values are {${a^{n+k} b^{n+k}}$} , {${a^{n+0} b^{n+k}}$},{${a^{n+k} b^{n+0}}$},{${a^{n+0} b^{n+0}}$}
from this string aab is accepted.
any idea?