For this case we propose a system of equations:
x: Let the variable representing the cost of a milkshake
y: Let the variable representing the cost of a burger
According to the statement we have:

We multiply the first equation by -5:

We multiply the second equation by 2:

We have the following equivalent system:

We add the equations:

Thus, the cost of a burger is $3.

So, the cost of a milkshake is $2
Answer:
Burger: $3
Milkshake: $2