How are these terms solved when in descending order?

Answer:
B
Step-by-step explanation:
in the expansion of (x+a)^n
(r+1)th term
[tex]T_{r+1}=ncrx^{n-r}a^r\\r+1=4\\r=4-1=3\\n=6\\T_{4}=6c3 x^{6-3}(-2)^3\\6c3=\frac{6*5*4}{3*2*1} =20\\T_{4}=20x^3(-8)=-160 x^3[/tex]