Use a system of equations.
Let b = price of a burger
Let f = price of an order of fries
12b + 9f = 57.75
3b + 2f = 14
Rewrite the first equation.
Multiply the second equation by -4 and write under first equation.
12b + 9f = 57.75
-12b -8f = -56
Add the equations.
f = 1.75
Now substitute f = 1.75 in the original second equation.
3b + 2f = 14
3b + 2(1.75) = 14
3b + 3.5 = 14
3b = 10.5
b = 3.5
A burger costs $3.50
Fries cost $1.75