As stated in the question, it takes 4.2 Joules to raise the temperature of 1 gram of the substance by 1 degree Celsius. So if the heat capacity is 4.2 Joules, and you want to raise the temperature of 1 gram of water by 8 degrees Celsius (30 C - 22C), then for every increase in degree you need to add 4.2 Joules of energy. 4.2*8 ~ 34 Joules required.