86. \(\dfrac{21^4}{3^4\cdot7^2}\).

\(21=3\cdot7\) ekanidan foydalanib, suratni ko‘paytuvchilarga ajratamiz:

\(\dfrac{21^4}{3^4\cdot7^2}=\dfrac{3^4\cdot7^4}{3^4\cdot7^2}=7^{4-2}=7^2=49\).

Javob: 49.