Me out ! will appreciate it so much

Start with the general formula. The key word is difference. So the general formula is 

D(x) = W(x) - R(x) Now substitute in the values for W(x) and R(x)
D(x) = 0.002x^3 - 0.01x^2 - (x^2 - 4x  + 13) Be very careful about the sign in front of the brackets.

D(x) = 0.002x^3 - 0.01x^2 - x^2 + 4x - 13 Do you see what that minus sign did? It affected all 3 terms.

D(x) = 0.002x^3 - 1.01x^2 + 4x - 13 Note: -0.01 - 1x^2 = - 1.01 x^2
That gives you a clear cut answer

C<<<<< answer.

That sign is the worst part of the question. Make sure you understand what it did.

The answer is 225 miles per 4.5 hours. hope this ; )