how do we represent the expression x+y'+z' in into one which can be represented using only NANDs (i.e. negations of products)

Question

I don't know how to do it, I've only been taught this partially and I need to do this so I can get bonus marks

I tried just negating the values but i don't think that's right. The answer I'm getting is x'+y+z which i feel is wrong. This is because on other websites I saw that they are doing stuff like 'not a and not b = a nand b' and i don't understand how to do it or if what they are doing is right.