So how do we know if 3/4 and 2/3 are proportional to each other? The core defining property of any proportion is that the product of the means is equal to the product of the extremes.
What does that mean? Well in simple terms it means A multiplied by D must equal B multiplied by C. You could also consider these ratios to be fractions and then simplify them down to their lowest terms and compare them. If they are equal, then they are proportional.
Let's first work out if A (3) x D (3) is equal to B (4) x C (2):
3 x 3 = 9
4 x 2 = 8
As we can see, 8 does not equal 9. so we can say 2/3 and 3/4 are not proportional