Cant be ljcox ,
because against an equation F and L have to be inverse square
Fr =1 /(2*pi*Sqrt(L*C))
Sqrt(L*C) = 1 /(2*pi*Fr)
L*C = 1/ (2*pi*Fr)^2
L = 1 / {((2*pi*Fr)^2)*C}
L= 1 / {4*3.141*3.141 *Fr*Fr*C)
If you are not sure of your answer, all you need to do is substitute any number to your variables in the original equation say 2 for L, 3 for C. From here, you can get fr. Now, from your derived equation, substitute the value of fr that you got, and 3 for C. If you get L equal to 2, your derivation might be right.
Note: These are not always true all the time. You have to use different values for your variables to verify your equation.