I have to find the area of the following by dividing it up into nine sub intervals.

$\displaystyle W(x)=-x^4/324+x^3/9-25x^2/18+7x, 0<x<18$

the width of each interval is 2, so nine sub intervals ought to be x=2,4,6,8,9,10,12,14,16,18......i multiplied the width by the f(2,4,6..etc) and the answer came out to be approx 178.96, however when i calculate the integral on tech the answer is 183.6....have i done something wrong?