Prove a uniform or non-uniform convergence of a functional sequence $f_n(x)=\arctan x^{3n} - \arctan x^{2n}$

Question

given $$f_n(x)=\arctan (x^{3n}) - \arctan (x^{2n}), x\in[0, +\infty)$$ prove uniform or non-uniform convergence on the intervals $$E_1=[0.75,1]; E_2=[1.5,2]$$

I know there's a technique here, but I do not understand how the intervals are accounted there.