In demorgan's theorem we were taught on three steps to achieve the answer
First step is to not all inputs
If the given is $ab$ then $(a)'(b)'$
Second step is to change the sign opposite to the current sign. From or to and and vice versa.
$(a)'+(b)'$
And lastly, simply not the final answer.
$((a)'+(b)')'$
If this is wrong please tell me. If not please proceed below.
So for exmaple the given is:
$ab + c$
Will I simply not the first term or not a and not b
$(ab)'$ or $((a)'(b)')$ ?
the fast answer is that you right (ab)'. if things like that confuse you the easiest way (at least for me) is to set e=ab and now you got e+c which you know to equivalent to (e'c')' and than you can exchange back to $$ (e'c')'=((ab)'c')'=((a'+b')c')'$$