Attachment 25749

Why is the coefficient of the x4 term stated like 0=1/3+B+D

I foil all of the terms and get the same result $\displaystyle x^4(A+B+D)$ but why is the equation 0=? Isnt the equality the x's coefficient(which is 1) equal to the variables?

Why is it 0=A+B+D??