Algebra Question Please Help

If   \(f(x)=3x-7\)     then find    

\(f^{-1}(f^{-1}(-1))?\)

Let

\(y=3x-7\\ \frac{y+7}{3}=x\\ x=\frac{y+7}{3}\\ so\;if\;\\ f(x)=3x-7\\ then\\ f^{-1}(x)=\frac{x+7}{3}\\ f^{-1}(-1)=\frac{-1+7}{3}\\ f^{-1}(-1)=2\\ f^{-1}f^{-1}(-1)=f^{-1}(2)=9/3 = 3 \)

f(x)=3x-7

f^{-1}(f^{-1}(-1))?

Here is a graphical representation of what is happening.