If the Sun were to become cooler (without changing its radius), the energy density at the surface

Question:

If the Sun were to become cooler (without changing its radius), the energy density at the surface would decrease according to Equation 8-56. Suppose the Sun’s temperature were to decrease by 5 percent. Compute the fractional change in the rate at which solar energy arrives at Earth. (Assume that the Sun’s surface is in equilibrium and radiates as a blackbody.)