А₁ = 10, a, 19.

It seems that you are referring to a sequence of numbers (or possibly the terms of a mathematical series) with 'a₁' representing the first term of the sequence. Given 'a₁ = 10' and 'a' as another term, followed by '19', it appears you have provided two specific terms and an unknown term in between.

To help you further, I need more information about the sequence. Is it arithmetic, geometric, or another type? If it's arithmetic, we need to know the common difference (d) or another term to determine it. If it's geometric, we need the common ratio (r) or another term to find it. If it's another type of sequence, please provide the pattern or rule governing the terms.

With the given information (a₁ = 10, a, 19), and assuming it might be an arithmetic sequence for example, if 'a' represented the second term (a₂) and 19 the third term (a₃), then we could find the common difference by subtracting the first term from the third term and dividing by the number of steps between them:

a₃ - a₁ = d(n-1)

19 - 10 = d(3-1)

9 = 2d

d = 9 / 2 d = 4.5

So, the common difference d would be 4.5. However, without additional information on what 'a' specifically refers to and the nature of the sequence, it's difficult to provide an exact answer. Can you please provide more context or details?