The answer here is that a soft drink costs $1.50 and fries cost $2. Here is how you can get that answer:
Let's let S be soda and F be fries.
We know that 2S + F = 5 because of your first statement. Because of your second statement, we know that 2S + 2F = 7.
So now we can substitute. In the first equation, let's find for F. F = 5 - 2S.
Now we can plug that into the second equation and we now have
2S + 2 (5 - 2S) = 7
2S + 10 - 4S = 7
-2S = -3
S = 1.5
Now we plug this value for S back into one of the equations.
3 + F = 5
F = 2