do these ratios form a proportion?

Answer:
No, because both of them will never be able to match; become equivalent. You would have to multiply 2 different numbers to make them match, meaning they would not be proportional. For a comparison to be proportional, the ratio you are trying to simplify to must be achieved by multiplying 2 numbers from the previous ratio by one LCD.
[tex]\frac{3\cdot 4}{7 \cdot 3} = \frac{12}{21}[/tex]
They are able to make 12/21, but that does not mean they are proportional.