The rows $n=6$ and $n=7$ in Pascal triangle have been kept vacant. Fill in the gaps.

Expand $(7a+3b{)}^6$.

Expand ${\left(\frac{-9}{2}a+b\right)}^7$.

Expand ${\left(a-\frac{7}{3}c\right)}^4$.

The number of terms in the expansion of ${\left(x^2-2+\frac{1}{x^2}\right)}^6$ is equal to $7.$

There is a term independent of both $x$ and $y$ in the expansion of ${\left(x^2+\frac{1}{y^2}\right)}^9$.