Change in pH + temperature + equilibruim problem?

need help with the following problem:

For neutral water the pH is 7.0 at 298K and 7.4 at 280K.



a) the value for Kw at 298 is 10^-14. what is the value for kw at 280K?

b) what effect does increase in temperature have on the equilibrium of dissociation of water?

c) From the data above, deduce whether the reation is exothermic or endothermic.



i think i can do b) and c) if i knew how to do part a). please help?Change in pH + temperature + equilibruim problem?
part (a)-



pH = 7.4 so [H+] = 3.98x10^-8 (pH = -log[H+])

[H+]=[OH-]

Kw = [H+][OH-]

Kw = (3.98x10^-8)^2 = 1.58x10^-15

Kw = 1.58x10^-15