First of all, apologies if this is in the wrong section It's a calculus problem but I'm stuck on the Algebra of it.

I'm supposed to find the critical numbers of f(x)=x^(3/4)-2x^(1/4).

I found the derivative to be f'(x)= (3/4)x^(-1/4)-(1/2)x^(-3/4), but for the life of me I can't remember how to go about finding it's zeroes.

I know one of them is x=0, and the book says another is x=4/9, but how do I get that? Thanks for any and all help in advance.