What is it? I think it's +6 Shuckle with assault vest in a sandstorm, but I'm not sure.
You are correct.
Shuckle with Maximum SpDef is 614.
Since 6 stages multiplies this by 4, you now get 2456.
Assault Vest multiplies this by 1.5, and this turns into 3684.
Also, in Sandstorm, SpDef is multiplied by 50% (1.5). This turns your number into 5526!
Here (Shuckle's base stat)
Here (Items, abilities and moves that increase SpDef)