Solución

a.   f(u)=u7f\left( u \right) = {u^7}, u=x7+7xu = {x \over 7} + {7 \over x}

b.   7(x7+7x)6(177x2)7{\left( {{x \over 7} + {7 \over x}} \right)^6} \cdot \left( {{1 \over 7} - {7 \over {{x^2}}}} \right)