The answer is 4.89
The explanation:
when the Kb of ((HOC2H2)3N) = 5.9 x 10^-7
1- we can get the value of Ka from Kb:
when Ka = Kw / Kb
= 1 x 10^-14 / 5.9 x 10^-7
= 1.69 x 10^-8
2- now we will calculate the [H+] value:
when [H+] = √(Ka*[HA])
=√((1.69E-8)(0.010))
= 1.3 x 10^-5 M
3- the final step we will calculate the PH value from the value of [H+]:
PH = - ㏒ [H+]
= - ㏒ 1.3 x 10^-5
= 4.89