First, we need to define k because it is a missing zero;
Knowing that a(2)=0;
a(t)=(t-k)(t-3)(t-6)(t+3)
a(2)=(2-k)(2-3)(2-6)(2+3)=0
(2-k)(-1)(-4)(5)=0
(2-k)(20)=0
(2-k)=0/20
2-k=0
2=k
Therefore, the final equation is a(t)=(t-2)(t-3)(3-6)(t+3) with the zeros being (2, 3, 6, -3)
Hope I helped :)