The operation $\diamondsuit$ is defined as:

$(a, b) \diamondsuit (c, d) = \frac{ac + bd}{ab - cd}$

Substitute $(a, b) = (4, 3)$ and $(c, d) = (1, 2)$:

$\frac{(4)(1) + (3)(2)}{(4)(3) - (1)(2)} = \frac{4 + 6}{12 - 2} = \frac{10}{10} = 1$