For the change in temperature of water we require heat which is given by formula
[tex]Q = m s \Delta T[/tex]
here we will have
[tex]Q = 4200 J[/tex]
[tex]m = 1 kg[/tex]
initial temperature = 90 degree C
final temperature = 91 degree C
now we will have
[tex]\Delta T = 91 - 90 = 1 degree C[/tex]
from above equation we will have
[tex]4200 J = (1 kg) s (91 - 90)^0C[/tex]
now we will have
[tex]s = frac{4200 J}{(1 kg)(1 ^0C)[/tex]
[tex]s = 4200 J/(kg ^0C)[/tex]