The expression "3 x 3/4" represents multiplication, which can be calculated as follows:
\[ 3 \times \frac{3}{4} = \frac{3 \times 3}{4} = \frac{9}{4} = 2.25 \]
On the other hand, "3/4 of ?" means you are looking for a quantity that, when multiplied by \( \frac{3}{4} \), gives a certain result. To find that quantity, you need additional information regarding what you want to calculate "3/4 of".
If you have a specific number for the "?" in mind, I can help you calculate 3/4 of that number. Please provide more details!