The water was heated by 10 ° C, consuming 4.20 • 10 ³ J of heat. Determine the amount of heat.

The amount of heat for heating the body is determined from the equation
Q = m * c * (T2 – T1), where
m – body weight,
c – specific heat of the substance,
a (T2 – T1) – temperature change.
m = V * ro, where
V – body volume,
ro is the density of the substance.
For water
s = 4200 J / (kg * deg)
ro = 1000 kg / m ^ 3

The amount of heat has already been given, perhaps you should find a lot of water?
m = Q / [c * (T2 – T1)]
m = 4200 / (4200 * 10) = 0.1 kg