H = m c ΔT

89.6 J = (20.0 g) × c × (40.0 - 30.0)°C

Specific heat of iron, c = 89.6 / [20.0 × (40.0 - 30.0)] J/(g°C) = 0.448 J/(g°C)

The equation:

Heat energy = mass * specific heat * change in temp

89.6 = 20.0*Spec heat *(40.0-30.0)

Specific heat = 89.6/( 20.0*10.0)

Specific heat = 0.448J/g.°C