If 1 kilogram of glass at 30℃ losses 800 joules, what is the new temperature?
energy = mass * (specific heat) * (temperature change)
800 J = 1 kg * (specific heat) * (30 ºC - T)
there are many types of glass , each with is own specific heat
... find a workable value and use it
... make sure the units include Joules and ºC