Let's start off by defining our first term as u⁰
u⁰ is unknown,u¹=3124,u² is unknown,ect..
Using the recurring formula which states;
u(n+1)=u(n)+d
u¹=u⁰+d
u⁰=u¹-d
u⁰=3125-d
Using the explicit relation, which states;
u(n)=u⁰+nd
u⁵=u⁰+5d
5=3125-d+5d
4d=-3120
d=-780
u²=u¹-780=3125-780=2345
u³=2345-780=1565
u⁴=1565-780=785
You answer is letter C