Please help me finish this problem

The error message that i get isCode:function bisect(n) syms x; f = 4 * x + 2; n = int32(n); err = 1; b=5; a=-5; while(b-a > err ) m = (a+b)/2 ; sa= sign(feval(f,a)) ; sb = sign(feval(f,b)); sm= sign(feval(f,m)); if( sm == sa ) a=m; else b=m; end end

??? Error using ==> feval

Argument must contain a string or function_handle.

Error in ==> bisect at 17

sa= sign(feval(f,a)) ;

Help me please.