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.