Ronald invested $68160 for 8 years and earned 6.2% interest compounded annually

If f(x)=3x+1 and f^-1=x-1/3, then f^-1(7)=

Quadrilateral abcd in the figure below represents a scaled-down model of a walkway around a historic site. quadrilateral efgh represents the actual walkway. abcd is similar to efgh. what is the total length, in feet of the actual walkway?

Can you me find the missing length. i attached an image.