Which monomial is a perfect cube?

49p9q3r24
81p12q15r12
121p9q3r6
343p6q21r6

Ok, so dis is wat yo do

xyz=(x)(y)(z)
x^3y^3z^3=(x^3)(y^3)(z^3)
split it up, if one is not a perfect cube, it is wrong
remember
(x^n)^m=x^(mn)
so 
(x^n)^3 is a perfect cube (for nicety, make sure that n is a whole number)
basically see which exponents are multiplules of 3

first one

49 is 7^2, not perfect cube
wrong

2nd one
81=3^4, not a perfect cube
wrong

3rd one
121=11^2,
wrong, not perfect cube
4th
343=7^3
p^6=(p^2)^3
q^21=(q^7)^3
r^6=(r^2)^3
343p^6q^21r^6=(7p^2q^7r^2)^3

answer is last one is perfect cube