1. Let's clarify the problem first: it seems there was confusion about the equation $y = a + 2d = 19$. 2. This equation likely comes from an arithmetic sequence where $y$ is the third term, $a$ is the first term, and $d$ is the common difference. 3. The formula for the $n$-th term of an arithmetic sequence is: $$a_n = a + (n-1)d$$ 4. For the third term ($n=3$), the formula becomes: $$a_3 = a + 2d$$ 5. If the problem states that the third term $y$ equals 19, then: $$y = a + 2d = 19$$ 6. This is not an arbitrary equation but follows directly from the arithmetic sequence formula. 7. To solve for $a$ or $d$, you need additional information or equations. 8. In summary, $y = a + 2d = 19$ means the third term of the sequence is 19, derived from the general term formula.