The circumfrence of any circle is 2πr.
The problem says that the circumfrence is = 6r+12.88ft
You just set these two equal to each other and solve for r.
2πr=6r+12.88 (subtract 6r from both sides to get r's on same side)
2πr-6r=12.88 (factor out the r)
r(2π-6)=12.88 (divide both sides by 2π-6)
r=12.88/(2π-6)
r=45.48ft
Your problem says to use an approximation of 3.14 for π. This will result in your answer being:
r=46ft