How is $x^\frac{9}{4} \cdot x^\frac{1}{2} = x^\frac{11}{4}$ How is $x^\frac{9}{4}  \cdot x^\frac{1}{2} = x^\frac{11}{4}$? 
I don't really understand it. 

$$x^{9/4}=(x^{1/4})^9.$$ 
$$x^{1/2}=(x^{1/4})^2.$$  Letting $y=x^{1/4}$, we get:
$$x^{9/4}\cdot x^{1/2} = y^9\cdot y^2=y^{11} = (x^{1/4})^{11}=x^{11/4}$$
