Factorize $9x^2 + 24x + 16$:

This fits $a^2+2ab+b^2$ with $a=3x$ and $b=4$, since $9x^2=(3x)^2$, $16=4^2$, and $24x=2\cdot 3x\cdot 4$. Therefore it equals $(3x+4)^2$.