Which of the following equations can be used to solve for the radius (r) ?

Formula: A = 3.14r^2

You did not include the equations that you want to assess whether they can be used to solve for the radius (r). Likely, the equation of the circumference, C = 2*Pi*r is included, if so = > r = C / (2*Pi). If you round Pi to 3.14, the equation may be written r = C / 6.28.