multiply x^1/3 times x^3/7
\(x^{\tfrac{1}{3}}\cdot x^{\tfrac{3}{7}}=\\\\ x^{\tfrac{1}{3}+\tfrac{3}{7}}=\\\\ x^{\tfrac{7}{21}+\tfrac{9}{21}}=\\\\ x^{\tfrac{16}{21}\)
