To visualize better, look at it like this:
[tex]\frac{3r^3}{2r}[/tex]
First, see if you can simplify 3/2. You can't, so leave it like that.
The rule for exponents is you subtract the exponent from the bottom (r) from the exponent on the top ([tex]r^3[/tex])
[tex]r^3 - r^1 = r^2[/tex]
Put that on the top, and you get
[tex]\frac{3r^2}{2}[/tex]
