Step-by-step explanation:

18 * 3.5 = 63
He wants to fry 63 potatoes
63/6 = 10.5
He will need 10.5 liters of oil to from potatoes for 3.5 hours.
Not sure how to put it into Matrix form but the answers would be.
11/2
-7
-19/2
and together that would be (11/2,-7.-19/2)
3/5n - 4/5 = 1/5n....multiply everything by 5 to get rid of the fractions
3n - 4 = n
-4 = n - 3n
-4 = -2n
-4/-2 = n
2 = n <==