Please explain the full equation

First two boxes are 6

Third box is -18

the last row is 3/1 (-18/1) = (1/3a) 3/1

Step-by-step explanation:

Ima try my best to explain:

-6 = 12 + 1/3a

+6    +6        

0 = 18 + 1/3 a    subtract 18 on both  in order to get 1/3a by itself

-18   -18

-18 = 1/3a          

-18/ 1/3  = 1/3a / 1/3 to make it easier multiply by the reciprocal on both sides

3/1 x -18/1 = 1/3 x 3/1      

multiply across will get you -54/1 which is just -54

to determine the unit cost your would divide the total cost by the number sold.

7 bagels for $4.41       $4.41/7 = $.63 each at fresh side

5 bagels for $3.60         $3.60/5 = $.72 each at value mart