Given the Following Acid Dissociation Constants:
K_a (H₃PO₄) = $\rightleftharpoons$

Given the following acid dissociation constants:
K_a (H₃PO₄) = 7.5 × 10^-3
K_a (NH₄⁺) = 5.6 × 10^-10
Determine the equilibrium constant for the reaction below at 25 °C.
H₃PO₄(aq) + NH₃(aq) $\rightleftharpoons$ NH₄⁺(aq) + H₂PO₄?(aq)

A) 4.2 × 10^-12
B) 7.5 × 10^-8
C) 4.2 × 10²
D) 1.3 × 10⁷
E) 2.4 × 10¹¹