help urgent

hi everyone, i need help on this problem

(a) Simplify

$$\dfrac{\dbinom{n}{k}}{\dbinom{n}{k - 1}}.$$

(b) For some positive integer $n,$ the expansion of $(1 + x)^n$ has three consecutive coefficients $a,$ $b,$ $c$ that satisfy $a:b:c = 1:7:35.$ What must $n,$  be?