Find a Formula For $(f \circ g)(x)$ Assuming That $f(x)=\log _{7} x$

Find a formula for $(f \circ g) (x)$ assuming that $f(x) =\log _{7} x$ and $g(x) =7^{7 x}$ .

A) 7x
B) 7
C) $\log _{7} x^{7 x}$
D) $7^{\log 7 x}$