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.

The square root of 18 is the only number that is irrational.
negative 10 over 2 is -5 which can be written as -5/1  or 5/-1  (rational)
-13.4 can be written as 13.4 over 1  (rational)
22.7... can be written as fraction:  22  7/9    or 205/9 if you prefer (rational)  

Only the √18 cannot be written as a fraction since it goes on and on without repeating.  There is no number times itself that equals exactly 18.