Question 42584
There are a lot of different ways to solve this problem. The best way I know to explain is as follows. 

I would change 2 and 1/2 to an improper fraction (big number on top) To do this you multiply the bottom number of the fraction times the whole number (or big number) and then add the top number of the fraction. The denominator stays the same. 

So 2 1/2 = 5/2

Then you have 3r/4 = 5/2. Cross multiply as you would a proportion. Multiply the bottom of the first number times the top number of the fraction on the other side. Vice versa with the other two.

So you would have 3r * 2 = 4 * 5 
which would equal   6r = 20    divide both sides by 6 to get r by itself.

                     r = 20/6 which would reduce to 10/3 
      You can change it back to a mixed number by dividing 3 into 10.  3 will go into 10 3 times with a remainder of 1. 

               So the answer would be r = 3 and 1/3
Hope I explained it well enough!