MATLAB中fzero()的问题~

MATLAB中fzero()的问题~
P = 9.33;
T = 300.2;
n = 2;
a = 4.17;
b = 0.0371;
R = 0.08206;
V=fzero('P*V^3-(P*n*b+n*R*T)*V^2+a*n^2*V-a*b*n^3',0)
这个程序错在哪?为啥说我没有给参数赋值?(我的是6.5的MATLAB)
Error using ==> fzero
FZERO cannot continue because user supplied expression ==> P*V^3-(P*n*b+n*R*T)*V^2+a*n^2*V-a*b*n^3
failed with the error below.
Error using ==> inlineeval
Error in inline expression ==> P*V^3-(P*n*b+n*R*T)*V^2+a*n^2*V-a*b*n^3
Undefined function or variable 'P'.
Error in ==> E:\MATLAB6p5\work\Untitled.m
On line 7 ==> V=fzero('P*V^3-(P*n*b+n*R*T)*V^2+a*n^2*V-a*b*n^3',0)