enum column change value by condition

[Selim_Tuncay]

i have two enum columns

1- enum column's options "a,b,c"

2-enum column's options "d,e"

i want to create a condition on 2.enum column is --if([1.enum column]="a", [_this],isblank[_this])---

but i can not get the result else!, i can not make the 2.enum column empty even i have changed 1.enum column to "b" or "c"

[Steve]

if([1.enum column]="a", [_this], "")

[Selim_Tuncay]

does not working 

i tried