How to add expire header to improve yslow grade

I have an F in add header and Im trying to improve the grade. this what I got so far and still got an F, Can somebody tell me why?
<IfModule mod_expires.c>
ExpiresActive on
ExpiresByType text/css “access plus 90 days”
ExpiresByType text/javascript “access plus 90 days”
ExpiresByType image/ico “access plus 90 days”
ExpiresByType image/jpg “access plus 90 days”
ExpiresByType image/jpeg “access plus 90 days”
ExpiresByType image/gif “access plus 90 days”
ExpiresByType image/png “access plus 90 days”
ExpiresByType text/css “access plus 90 days”
ExpiresByType text/html “access plus 90 days”
</IfModule>