Unidentified Function error when trying to solve an equation

I am simply trying to solve the equation for the variable k and every time I run the program it comes back with an unidentified variable error below is the code and the result

When I copy it into wolframalpha it comes back just fine so I know I didnt type the equation wrong.

Code

solve(2*pi*sqrt((.5*((k^2)+(((4/3)*k)^2))))==100 , k)

Result

Undefined function or variable 'k'.

Error in MitchAndrewsLab1 (line 18) solve('2*pi*sqrt((.5*((k^2)+(((4/3)*k)^2))))==100' , k)

Answer 1

[Sean de Wolski]

You need to define k as symbolic before the expression:

syms k
solve(etc,k)

[Mitch]

Thanks so much! I figured it was something simple like that.