When computing $\gcd(21, 15)$ using the extended Euclidean algorithm:

• $21 = 15 \cdot 1 + 6$
• $15 = 6 \cdot 2 + 3$
• $6 = 3 \cdot 2 + 0$

Using back-substitution to express $3 = 21x + 15y$, what is the value of $x$?