Simplify the expression x4/3y5/21x2y8/7x2/3\displaystyle \frac{{{x}^{{{4}/{3}}}{y}^{{{5}/{21}}}}}{{{x}^{{{2}}}{y}^{{{8}/{7}}}{x}^{{{2}/{3}}}}}. Write your answer in rational exponent form,using nonnegative exponents.