What Is the [H₃O⁺] in a Buffer That Consists of 0.30

Multiple Choice

What is the [H₃O⁺] in a buffer that consists of 0.30 M HCOOH and 0.20 M HCOONa?
K_a = 1.7 $\times$ 10^-4

A) 1.1 $\times$ 10^-4 M
B) 2.6 $\times$ 10^-4 M
C) 4.3 $\times$ 10^-4 M
D) 6.7 $\times$ 10^-5 M
E) None of these choices is correct.