The GCD of given numbers is 19.
Step 1 : Find prime factorization of each number.
$$\begin{aligned}209 =& 11\cdot19\\[8pt]551 =& 19\cdot29\\[8pt]\end{aligned}$$(view steps on how to factor 209 and 551. )
Step 2 : Put a box around factors that are common for all numbers:
$$\begin{aligned}209 =& 11\cdot\color{blue}{\boxed{19}}\\[8pt]551 =& \color{blue}{\boxed{19}}\cdot29\\[8pt]\end{aligned}$$Step 3 : Multiply the boxed numbers together:
$$ GCD = 19 $$This solution can be visualized using a Venn diagram.
The GCD equals the product of the numbers at the intersection.