You are given the initail and final temperature of the substance which is 15°C and 40°C with a mass of iron of 100 grams. Also the specific heat of iron is 0.46 J/g°C. You have to use the formula Q = mcpΔT to find the heat, Q, cp (specific heat of substance) and ΔT the change in temperature.
Q = mcpΔT
Q = 100*0.46*(40 - 15)
Q = 1150J