To check if a proportion is true, we simply have to use "cross-multiplication", that is in our case multiply the second denominator times the first numerator, and the second numerator by the first denominator, and see if we get an equality:
3 / 6 = 4 / 8
Then cross multiplying:
3 * 8 = 4 * 6
24 = 24
We can confirm that this proportion is TRUE because we got the same value on the left and on the right of the equal sign.