Answer:
Yes they are proportional!
The factors of 28 that we can use in these fractions are 4 and 7. To find if these are proportional, just simplify 8/28. To do so divide the numerator (top) and the denominator (bottom) both by 4...
8/28 / 4 = 4/7
Now that you have both with the same denominator, you can evaluate if 3/7 and 4/7 are proportional. In this case they are.
Hope this helps! :)