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. 

Use the exponent rule $x^a\cdot x^b = x^{a + b}$ with $a = 9/4$ and $b = 1/2$. Since $b = 2/4$, $a + b = 9/4 + 2/4 = 11/4$.
