How to make a subtraction between 2 records from the same column using rails?

Hi Stackoverflow team.

I'm using ruby on rails and i'm trying to find the code to make a subtraction between 2 records from the same column using rails.

I have the following table

customers
    id    num_hid     num_oxi
    1        1           2
    2        3           4
    3        5           6
    4        7           8

I'm trying to get this result

 @result =  last_num_hid_id_4 -  last_num_hid_id_3
 //calcultion is: 7-5 = 2

i did this code but this displays the last 2 records in array

<% @customers.each.last(2) do |customer|%> %>
  <%= customer.num_hid %>
<% end %>

i tried this code but is only showing the last record

@last_customer =Customer.all.last
@last.num_hid

Can somebody help me with this issue?

I will appreciate all your comments.

Thanks in advance.