a3=2+2d

a6=2+5d

In geometric series, then

a3^2=a 1*a6

(2+2d)^2=2(2+5d)

4d^2+8d+4=4+ 10d

4d^2-2d=0

2d(2d- 1)=0

D= 1/2 (because d is not 0)

an = a 1+(n- 1)d = 2+(n- 1)* 1/2 = 3/2+n/2

sn=(a 1+an)n/2=(2+3/2+n/2)n/2=7n/4+n^2/4

I hope my answer is helpful to you. Take it, O(∩_∩)O!