Q:  If f(r)=3 [r/(3^1/2)]² (3r ^-1)^-1
simplify this expression and find df/dr.

A:  3 [r/(3^1/2)]² (3r ^-1)^-1 = 3 [r²/3] r/3 = r³/3.  Thus df/dr = 3r²/3 = r². 

Answer to 2 point bonus question about the Simpsons: 
After Bart switches IQ tests with the smart kid, his new teacher at the school for talented and gifted kids gives this a problem and all of the other kids get the joke.  Notice that df = r² dr or r dr r (har de har har).