Answer: 
Step-by-step explanation:
We have the following information:
Fredburguer  , milkshake
, milkshake  and an order of fries
 and an order of fries  costs
 costs  :
:
 (1)
 (1)
One Fredburguer is equal to a milkshake plus orders of fries:
 (2)
 (2)
Three milshakes is equal to a Fredburguer plus an order of fries:
 (3)
 (3)
And we are asked: If we have a Fredburguer, a milkshake and two orders of fries, how much do we have to pay?
 (4)
 (4)
Well with the first three equations we have a system of equations to solve. Let's begin by isolating  from (3):
 from (3):
 (5)
 (5)
Making (2)=(5):
 (6)
 (6)
Isolating  :
:
 (7)
 (7)
Substituting (7) in (2):
 (8)
 (8)
Isolating  :
:
 (9)
 (9)
Substituting (7) and (9) in (1):
 (10)
 (10)
Isolating  :
:
 (11) This is the cost of a Milkshake
 (11) This is the cost of a Milkshake
Substituting (11) in (7):
 (12)
 (12)
 (13) This is the cost of an order of frieds
 (13) This is the cost of an order of frieds
Substituting (11) in (9):
 (14)
 (14)
 (15) This is the cost of a Fredburguer
 (15) This is the cost of a Fredburguer
Substituting (11), (13) and  (15) in (4):
 (16)
 (16)
Finally:
 This is the cost of a Fredburguer, a milkshake and two orders of fries
 This is the cost of a Fredburguer, a milkshake and two orders of fries