If sum of four numbers in A.P. is 28 and product of two middle terms is 45, then product of the first and last terms is

Let four numbers in A.P. bea−3d, a−d, a+d, a+3dWe are given             28=(a−3d)+(a−d)+(a+d)+(a+3d)=4a⇒      a=7Also, (a−d)(a+d)=45⇒ 49−d2=45⇒d=±2.Now, (a−3d)(a+3d)=a2−9d2=13