test "if statement"

I want to do "if δ>0 then k=a+b else k=c+d"

But impossible to include the equation k:=a+b in the "if statement" below

When I type "equal" after "k" the "equal" is put on the fist line and answer "k-not defined"

I found an exemple on the forum but I'm not able to reproduce it by myself

Exemple found on the forum