How do you factor a^2+20a+64?

$\left(a + m\right) \left(a + n\right) = {a}^{2} + \left(m + n\right) a + \left(m \times n\right)$
Notice that $20 = 16 + 4$ and $64 = 16 \times 4$
So ${a}^{2} + 20 a + 64 = \left(a + 16\right) \left(a + 4\right)$