When you do these operations you start putting the amount there giving to you 91 moles of CuO. Like this 91 moles CuO
Because it’s moles to moles you just need to put the mol ratio of the operation, the mol ratio is B/A
B=Unknown (the work is asking how many moles of N2 can be made so you don’t now it yet their for it’s the unknown
A=Known(the work is saying 91 moles of CuO so it’s saying how much so thats why it’s the known)
So u have to put the moles of the unknown and known but you didn’t include the chemical reaction in the question so depending in if you understand this you don’t need to or if you didn’t understand tell me what is the chemical reaction, im going to insert a work sheet of this work so you can understand better